YUI CSS. I added it into one of my side-project web apps last night (counting the time to actually read the docs).
Dead simple, and doesn't require using any of the YUI javascript if you don't want to.
ctrl++
Is there a firefox and/or IE javascript hook that simulates control- and control+? I mean a way to have a user click a button to resize all fonts, not just switching between predefined font sizes in style sheets.