[Windows 10 Mobile] AshleyT:
This thread is a sort of recap of what we're able to do on Windows 10 Mobile.

I will regularly update this thread with new content as soon as possible.

Device Unlock

vcREG: lumia reg editor + interop unlock. Now Supports X50 device interop unlock

Description: vcREG is a powerful Lumia registry editor developed by vcFan for Lumia Windows Phones that has SYSTEM level access to the registry. the tool also has the ability to unlock all app capabilities for third party apps(interop unlock), and give SYSTEM level access to most of the filesystem through MTP.

[1/1/2016]Permanently interop Unlock your WP (even after hard reset)

Description: This thread by Riyad_ explains how to manually unlock your Windows Phone permanently, even after an hard reset. You can also use vcREG 1.2 to obtain the same result automatically.

[JAILBREAK][GUIDE]Interop Unlock for Windows 10 Mobile + All Capabilities

Description: Root Tool v2.0 is a tool developed by djamol that allows you to Interop/Capability unlock your Lumia/Samsung device. It works fine on build 10586 but does not work with the latest update of extras+info. If you have already updated extras+info just hard reset your device, disable automatic store updates and unlock your device (or just use vcREG 1.5).

Windows Phone Internals - Unlock bootloader, enable Root Access, create Custom ROM

Description: This tool by Heathcliff74 allows you to unlock the bootloader of selected Lumia Windows Phone models. After unlocking the bootloader, you can enable Root Access on the phone or create and flash Custom ROM's. Root Access allows you to load your own homebrew software onto the phone with high privileges. Apps can escape from their sandboxes. The tool can also create backup-images of the phone and access the file-system in Mass Storage mode.

Guides

[TUTORIAL] Deploy XAPs easily with WPV Xap Deployer

Description: This guide by MrCego explains how to sideload XAP files on your device using WPV XAP Deployer and Windows Phone SDK Lite. NOTE: If your device is not yet unlocked, you need to enable "Developer mode" in Settings > Update & Security > For Developers.

Tutorial | Lumia devices flashing

Description: This guide by Kaptaiin explains how to flash your Lumia device with a stock ROM. This guide is recommended to people who want to flash a specific ROM (e.g. to debrand or downgrade). If you simply want to flash the latest stock ROM consider using Windows Device Recovery Tool.

Steps to Download Windows 10 Mobile version of Apps manually ( Appx files )

Description: This guide by katsuga explains how to download appx files from the store on your computer.

[GUIDE] Unlock lumia x2x and enable Mass Storage mode

Description: This guide by ADeltaX explains how to enable mass storage mode (not MTP) on Lumia x2x devices (the ones supported by WPInternals).

windows mobile 10 Redstonebuild 14627& higher for old lumias in insider fast Ring

Description: This guide by raghulive explains how to install Windows 10 Mobile 14627 (Redstone) Insider Preview on discontinued Lumia devices in the Insider Fast Ring.

[GUIDE] Full FileSystem Access over SFTP on Windows 10 Mobile

Description: Full FS Access via MTP doesn't truly give full file system access as there are files and folders that are still inaccessible. Thanks to this guide by snickler you'll be able to browse the whole filesystem through an SFTP connection. Works fine on build 10586, just ignore wconnect errors.

[TWEAK] De-watermark Bing Lockscreen

Description: This guide by winphoneuser explains how to remove the Bing watermark from the "Pic of the day" when you set Bing as lockscreen wallpaper.

[TUTORIAL] Changing the font system (Buxton Sketch, Comic Sans and more)

Description: This guide by denisf1981 explains how to manually change the system font. You can also do this with DHT Font Change (see Utilities).

[HOWTO] [CAUTION] Maximize the potential of the (Lumia) Win10M camera

Description: This guide by AsvinDG explains what registry keys you should edit to maximize the potential of your camera (Lumia only).

[TUTORIAL] How to change the icons of quick status of the lock screen.

Description: This guide by denisf1981 explains how to replace lockscreen icons for any application. Interop-Unlock and Full FS Access are required.

[W10M BUILDS] Update links collection (cab-files) for various devices

Description: This thread by HD2Owner explains how to manually update to a specific Windows Insider build. To update, you need cab files specific for your device. Only few devices cabs are listed.

[TWEAK] Install homebrew apps as settings apps

Description: This thread by winphouser explains how to list a sideloaded application as a setting app.

URIs for URI Launching items

Description: This thread by SammaelAkuma explains how to create shortcuts to settings, app features and more using an URI (Uniform Resource Identifier). You will need to know the URI address of the feature you need. No root or unlock needed.

Utilities

[Firmware] Download all Lumia firmware And emergency files

Description: This website by Cholens lists the latest firmwares for all the Lumia devices available on Microsoft servers.

[XAP][18/09/2015][0.6] CustomPFD: registry editor + tweaks

Description: A great utility developed by Pasquiindustry that allows you to browse and edit part of the system registry. There are also lots of settings that you can customize directly without the need of browsing the system registry.

[App] App Data Manage Tool - Create backups from current state of apps and games

Description: This tool made by mahdi75 allows you to backup data of any app installed on your phone and restore it later, even on another phone. It can also erase the storage of your apps which is like reinstalling them but faster.

[App][Source] DHT Upgrade Advisor (Get redstone on old lumias)

Description: Using this app developed by Dormant Hackers Team, you can upgrade any unsupported Lumia device to Windows 10 Mobile Anniversary Update.

[APP] Interop Tools - A versatile registry editor, registry browser, unlock and more!

Description: This app by gus33000 allows you to edit the registry through the registry editor, browse the registry hives through the registry browser, find sepecific registry values/keys through the search page, apply tweaks to your device, Interop and Cap unlock your device, access device information, remotely edit the registry through the built-in desktop app, manage applications, manage certificates, perform standard registry operations such as deleting, renaming, and copying details, built-In Command Prompt inside the app, manage SSH accounts and more.

[116 TWEAKS] 2015-09-30 Tweaks for Windows 10 mobile - Quick Action button etc.......

Description: This tool is developed by kwanice and is based on WPTweaker by sensboston. It allows you to customize lots of settings to tweak your phone like the ability to use static IP, add 5th action button, etc.

[XAP]▒▒▒▒▒ Storage Explorer ▒▒▒▒▒

Description: A file manager developed by spavlin that allows you to browse, copy, edit and delete files from the root of the File System. Not all the folders are accessible. Some folders are accessible by writing the full path. If you have unlocked your phone with WPInternals check out post #37 for a full FS access version of Pocket File Manager and post #40 for a full FS access version of Files by Microsoft.

[XAP][Source] DHT Font Change for Windows Phone

Description: This app developed by DHT group allows you to replace the default system font with a custom one. It officially works on Lumia devices, Samsung devices are partially supported.

[XAP] AdBlocker For Windows Phone 8.x+

Description: This app developed by sandix allows you to replace HOSTS file to block annoying ads. You can customize the URL of the HOSTS file with multiple entries, view the entries of your current HOSTS file and restore your HOSTS file with an empty one.

[APP][2.0] Device Diagnostics HUB: Task Manager and more!

Description: This app by Pasquiindustry allows you to get lots of useful information about your device (CPU/RAM/GPU usage, download/upload speed, active system processes etc.) using Devices Portal.

Discontinued

[Fix added] [HOW TO] [UPDATED] Install Android apps on Windows 10 Mobile

Description: This thread by XDRdaniel explains how to deploy apk files on Windows 10 Mobile. Unfortunately Windows Bridge for Android (aka Project Astoria) is not available anymore since build 10549.

[GUIDE] Deploying Astoria Packages Manually To W10M

Description: This thread by HD2Owner explains how to deploy cab files to enable Windows Bridge for Android on unsupported devices. Unfortunately, as above, Windows Bridge for Android (aka Project Astoria) is not available anymore since build 10549.

[How to]Root Project Astoria

Description: This thread by ADeltaX explains how to obtain root access on Project Astoria. At the current stage, apps can't request root access because of the limitations of Astoria. The project is currently inactive because Astoria is not part of Windows 10 Mobile since build 10549.

[TOOL][OPEN SOURCE][GOOGLE][ALPHA] AM GApps Tool: GApps to W10M

Description: This thread by Marocco2 explains how to install GApps on Windows 10 Mobile. Currently, the tool has some issues (GMS not working), so it isn't fully functional. The project is currently inactive because Astoria is not part of Windows 10 Mobile since build 10549.

HOWTO: Install Windows 10 for Phones on Non-supported devices (and other hacks)

Description: This thread by RustyGrom explains how to install Windows 10 Mobile on unsupported devices using FiddlerCore. This method is reported to not work anymore after an update of Windows Insider app.

[W10M] Get Skype picture sharing from redstone (14267) working on 10586.107

Description: This guide by HD2Owner explains how to get Skype picture sharing on build 10586.107. Not useful anymore since the update has been officially released also for TH2 devices.