Yahoo User Interface
Yahoo has been kind enough to make all the code they use for designing their sites available to the public, from funky JavaScript stuff to core css. I use the fonts css to normalize the font on this site, looks nice and is a damn sight better looking than the default browser style.
Something I am going to play with is the grids system which lets you split up the page into sections with preset code. Looks really cool, will probably use it for making a theme from scratch for this site (for real this time). I’m pretty crap at explaining how it works so here’s a video from one of the developers for Yahoo.