planned now available (Android and IOS support are not mentioned). The NXT Solaris client uses the LkJSON library which doesn’t look like it is cross platform yet. NextCoin according to this page is a descendant of Bitcoin but is brand new software not not a fork of the original source code. It is a proof of stake currency where coins are earned by charging transaction fees. Mining in NextCoin doesn’t cause monetary inflation. NextCoin claims to have many improved features over Bitcoin including a resolution for the big environmental issue.
Update: The Mac OSX version source code is also now available.