[AT&T Samsung Galaxy S II SGH-I777] CyanogenMod:
CyanogenMod is a free, community built, aftermarket firmware distribution of Android 2.3 (Gingerbread), which is designed to increase performance and reliability over stock Android for your device.

#include <std_disclaimer.h>
/*
* Your warranty is now void.
*
* I am not responsible for bricked devices, dead SD cards,
* thermonuclear war, or you getting fired because the alarm app failed. Please
* do some research if you have any concerns about features included in this ROM
* before flashing it! YOU are choosing to make these modifications, and if
* you point the finger at me for messing up your device, I will laugh at you.
*/

CyanogenMod is based on the Android Open Source Project with extra contributions from many people within the Android community. It can be used without any need to have any Google application installed. Linked below is a package that has come from another Android project that restore the Google parts. CyanogenMod does still include various hardware-specific code, which is also slowly being open-sourced anyway.

All the source code for CyanogenMod is available in the CyanogenMod Github repo. And if you would like to contribute to CyanogenMod, please visit out Gerrit Code Review. You can also view the Changelog for a full list of changes & features.

Instructions:

Note: this build as a Release Candidate has been superceded by the latest nightlies due to many bug fixes. This thread will get a new Release Candidate as soon as we feel it is ready for Release.

First time flashing CyanogenMod 7 to the Samsung Galaxy S II, or coming from another ROM?

Root the device and install ClockworkMod Recovery. Instructions are available here.

Perform a NANDroid backup of your current ROM.

Format the system, data & cache partitions of your device.

Perform a factory reset.

Flash CyanogenMod.

Flash CyanogenMod again (without reboot - this is a bug we will fix soon).

Optional: Install the Google Apps addon package.

Upgrading from earlier version of CyanogenMod 7?

Perform a NANDroid backup of your current ROM.

Flash CyanogenMod (your Google Apps will be backed up & restored automatically).

Issues?

Experience issues? Please provide the following info:

If the device was hard reboot, please provide the file "/proc/last_kmsg".

If the device was soft reboot or is "bootlooping", please run a logcat and provide the full output.

Please use Pastebin when possible.

Download Links:

Official post​

Upon review of user feedback and bugs that have been discovered with what was released as 7.1.0 stable, we no longer feel the 'stable' tag is appropriate for this rom.

The download is still available, however it is now being treated as a Release Candidate, rather than a proper stable release.

Sorry for all the confusion, and any issues you may experience. We will release a 7.1+ once we feel the ROM has matured to a truly stable status.

-CyanogenMod SGS Team​

CyanogenMod:

Latest version: update-cm-7.1.0-RC1-Galaxys2ATT

Download: link

Mirror: link

MD5sum: aab37aa86f6615f0ddc7a717022cc141​

Google Apps addon:

Version: gapps-gb-20110828

Mirror: link

Mirror: link

Version: Google Talk with video addon

Mirror: link

Mirror: link​

The CyanogenMod team would like to thank everyone involved in helping with testing, coding, debugging & documenting! Enjoy!