Alongside CyanogenMOD, MIUI is one of the most popular “alternative” ROMs in the Android space. Of course you must have the knowledge (and the right phone) to flash custom ROMs and kernels, but the Nexus S was born for that kind of stuff.
MIUI, developed independently in China, is one of the most drastic UI overhauls we’ve ever seen on Android. They’ve even been so successful that some Android devices are shipping with it out of the box. Now, the team has used the Android 4.0.1 source code to update their ROM to an Ice Cream Sandwich base. While it looks much like the Gingerbread version it’s all new under the hood.
Right now the Nexus S is the only device that supports the new version but, as with CyanogenMOD, that number will quickly increase.