If the technology itself wasn’t cool enough already, check out that generous BSD license — “free for all uses.” Getting your hands on the Y! UI Library is incredibly straightforward, too. I just downloaded the zip file and the zip file unzipped with no funny business.
I always like playing with real examples, and there are plenty of those (these are just a few that caught me eye — there is much more and all these are backed up by detailed documentation):
All I can say is: have fun.