Twitter Bootstrap
It's a bit of a shame that the "just hotlink the css" thing doesn't actually work very well - it actually means that some parts won't function (such as the Dropdown menu). My partner and I have just spent the better part of half an hour trying to work out why this didn't work (the documentation didn't mention any javascript necessity).Docs could use some serious clarification - which parts are CSS-only and which require what js files.