Embarcadero's new RAD Studio XE5 introduces unified, single source cross platform Development for Win32, Win64, Mac OS X, iOS and Android. This is currently the best way to write cross platform applications over all these platforms. Certainly Mono, can work to hit most of them; but deployment of such apps along with their performance is extremely complicated. RAD Studio provides native level code on all of these platforms. At some point, I hope to post more on this in detail.
In addition, they've implemented a new REST Client API supporting the REST Protocol and Open Authentication 1.0/1.1 and Open Authentication 2.0 this allows easy connection to most of the modern web API's.
Unfortunately there is a bug in the OAuth1 client that causes OAuth1 authentication to fail in the western hemisphere for Twitter, Tumblr and other 1.1 API's I've tried.