Another fork of Android that makes sense is Firefox OS. They take the Android kernel and drivers to get hardware compatibility, and layer a new set of APIs and apps on top of it. They don't care about compatibility, because they're interested in promoting web technologies rather than selling hardware, promoting services or making money from licensing.
I wouldn't exactly call Firefox OS an Android fork. Android is more than just the Linux kernel, it's also Dalvik and a large set of Java APIs. From an app developer perspective, the language and APIs matter much more than the kernel; Firefox OS is completely different from Android in this regard.
