CyanogenMod is a free, community built, aftermarket firmware distribution of Android 4.0 (ICS), which is designed to increase performance and reliability over stock Android for your device. CyanogenMod is easily the most popular AOSP (Android Open Source Project) ROM out there today.
Thanks to the great work by the xda developer “codeworkx”, the current CyanogenMod 9 RC for Galaxy S2 (I9100) is fully working ROM and you can use this as a daily driver. Obviously this is a Release candidate, not final yet as in all the nightlies. Updates will be developed on a regular basis until to the final version.
Check out the video review of the latest CyanogenMod 9 :
If you want to begin the adventure, you can directly download from the site of Cyanogen along with Google Apps. If you need any help regarding the installation, check out the latest FAQ or the known bugs.