Seems to be the best way to build cross platform mobile apps around. It's open source, free, and includes a cloud based build platform that will build an app for multiple mobile environments all "in the cloud". Easy.
Funky open source web app around task management that syncs and has a client across Mac, Win, iPhone, Android. Apparently no linux support. Built on top of Titanium from appcelerator.
Incredibly interesting app development platform that builds apps for all platforms (desktop & mobile, including linux, yay!) quickly with a single API. Open source, free, pay for premium support. Very, very interesting business.