[HTC Sensation] digiholic:
This guide was originally just for LCD density tweaks but it is also a good guide on how to edit your build.prop file using two methods (one paid app one free app). For a great list of other configuration settings you can work with head over to this thread.

Super Build.prop Tweaks

With our qHD resolutions we have the ability to easily read very small text or see small images clearly. Take that to the next step and decrease your LCD density and you can make things even smaller on the screen while still being clear and legible. Here is how you do it.

You WILL need to be S-OFF and permanently rooted. Click here for ikhzter's excellent guide for obtaining S-OFF and root. AND ALWAYS DO CLOCKWORK BACKUPS!

Spareparts is recommended. Get it here from the market. It's not needed for just editing your build.prop but it's handy for any Sensation owner.

You will also need a file explorer application that allows you R/W access. I use Root Explorer. It's not free but cheap and probably the best one out there. ES File Explorer is free and it supports R/W and root level access but it's a beta function. It does seem to work fine. Both options will require superuser access so allow it if you get the prompt. When you load the application you should see a message at the bottom "xxx has been granted superuser permission".

Root Explorer Method

Step 1: Open Root Explorer ("allow" superuser if first time loading) and you should be at the root partition. / Scroll down and select "system". You should now be located in /system.

Step 2: Select the "Mount R/W" button at the top. The "r/o" to the left of the button should change to "r/w".

Step 3: Long press build.prop and "Open in Text Editor".

Step 4: Scroll about half way down and find the line "ro.sf.lcd_density=240". Change it to a value you like 167/180/200/220. (I'm using 187 right now).

Step 5: Select the "menu" button on your phone and then select "Save Changes". Root Explorer will make a nice build.prop.bak backup for you.

Step 6: Open the Spare Parts application and make sure "Compatibility Mode" is unchecked.

Step 7: Reboot your phone.

ES File Explorer Method

Step 1: Open ES File Explorer and select your "menu" button on your phone then select "settings". Scroll down and check "Root Explorer" and select "yes" for the experimental feature. Superuser will pop up so "allow" it and check "remember". Check "Mount File System". Select your "back" button on your phone.

Step 2: You should be at the sdcard partition. /sdcard Select the sdcard image in the top left and it should change to a home image. You should now be at root. / Scroll down and select "system". You should now be located in /system.

Step 3: Press build.prop and select "ES Note Editor".

Step 4: Scroll about half way down and find the line "ro.sf.lcd_density=240". Change it to a value you like 167/180/200/220. (I'm using 187 right now).

Step 5: Select the "menu" button on your phone and then select "save".

Step 6: Open the Spare Parts application and make sure "Compatibility Mode" is unchecked.

Step 7: Reboot your phone.

If you changed to something like 167 or 180 you should easily see the change as many things are now much smaller. If you don't like the results you can try changing to a different value. Just reboot after each change. BE VERY CAREFUL trying a very low value. 167 is probably too small so try that first if you want to be extreme. I have tested 167/180/200/220. They all work but some things might seem a little weird. Still testing mine and seeing what I like best.

-Testing Update-

Looks like Sense related stuff can have issues. I've had the Sense launcher not work at all for me, but work after factory reset with LCD density still changed. Some Sense pieces scale and some don't but still work. If you use the Sense launcher LCD density changes might produce odd results.

Non-Sense (not HTC created) related things like alternative launchers or lockscreens, apps, etc work great so far. This guide will become more relevant after ROMs like CM or MiUi come out.

If you mess up your build.prop and can not get into Android read here:

In a command prompt:

- adb pull /system/build.prop

- edit the file

- save the file

- adb shell mount -o remount rw /system

- adb reboot