The biggest group creating custom Android versions, or mods, is Cyanogenmod, and now they’re going commercial. They’ve got venture capital and they’re already working on things that they didn’t have time for when it was just a hobby.
This is very cool in many ways. Right now, nearly everyone (in the US anyway) gets their copy of Android from one of the big carriers. These companies 1) install crapware on the phone that you can’t remove without rooting it, and 2) usually don’t provide upgrades of Android, even when they would run just fine on the phone.