I have this in my guide post but thought it needed its own post. If you have a capped data plan then you definetely need to check this out.
It has basically turned my 4GB plan into a 20GB plan!
The SuperUser's Guide: How To Extend Your Data Plan
Onavo Extend lets you get more out of your data plan, so you can do more web browsing and apps, consume more music, video and photos on 3G without breaking your data cap.
After you install Onavo Extend you can go back to using your phone or tablet as before. In the background, data usage from all apps and web browsing is compressed so that you do more with your Androidís mobile data month after month. And, if you travel abroad with your phone, the money you save could be substantial.
IF YOU'VE INSTALLED A CUSTOM ROM:
Some ROMs and kernels are missing important networking support that Android 4.0 APIs, and Onavo Extend, rely on.
If you've received a message saying your version of Android is unsupported, or if the application always says "service paused", please contact us via the "Feedback" menu item so we can help.
Trying a different ROM or kernel should help as well
Be sure to post your results, and what ROM you are using!
This is confirmed working on AOKP!