jQuery
I've got an unnatural obsession with jQuery at the moment - and it's not just because of the lowercase j and the uppercase Q, which seems to be the 'thing' since iPod's. I know it's just another JavaScript library like MooTools or Scriptalicious or whatever, but this one seems different. Not only does it have some kick ass functionality that the likes of Apple and the BBC are making use of... but it's quite easy to get your head around.
I've been messing around with jQuery for a while now - but I've recently discovered Web Designer Wall's tutorial 'jQuery Tutorial for Designers'. If you're a web designer, and you have any interest whatsoever in being on top of your game - read this!