Bootstrap CSS/HTML library from Twitterhttp://twitter.github.com/bootstrap/
I've been playing around with Bootstrap and I'm impressed. It makes it really easy to create elegant web pages and web UI's. For most of the functionality you just need to include one CSS file and then apply a single style to an element for each effect. It includes responsive grids, layouts, buttons, icons, forms and lots more. Bootstrap is open source under the Apache licence, which means it can freely be used in commercial apps/sites.