CyanogenMod offers a customised open-source firmware version of Android for a variety of phone and tablet hardware. It provides some features not available in the vendor-supplied firmwares, it's more customisable (if you want, you can build CyanogenMod for yourself), and it allows you to avoid the bloatware that comes with vendor firmware. It also tends to give quicker access to updates. There are a few more reasons, too, why you might want to try CyanogenMod; for me, the opportunity to lose a particularly annoying piece of proprietory software on my Galaxy S2 was a big gain. Read on for my experience installing CM on a Galaxy SII.