Mozilla announced last year that they were working on a mobile operating system based on web technologies (HTML, CSS, JavaScript, etc). Work has been pogressing on it and it was demoed at MWC 2012.

Support was mainly restricted to Samsung Galaxy S II phones but recently support for Nexus S was landed in the source repository. I built it and tried it on my Nexus S to see what it was like:

Nexus S running B2G

Using a Vodafone prepay SIM I called make/receive calls and send/receive texts. The web based games mostly work - cut the rope for example worked fine. And my own JavaScript 8080 emulator ran space invaders and good speed. The Nexus S hardware support is fairly new so camera doesn't work and there's the odd hang and crash but it's looking promising. I like the user interface. You can even "View Source' on the main screen and see the HTML for it.

Plus, it's open source and completely hackable. Nice.