I recently came across Bootstrap (from Twitter) and have used it now in a few different webapps. Needless to say, I have been quite impressed with the quality of this CSS framework and it’s ability to allow developers to rapidly mock up a user interface for their applications. The only downside is that a lot of your apps will have a similar look and feel, but can rapidly be customized by layering on some more customized CSS to provide a unique look and feel while leveraging a lot of the great work that has gone into Boostrap.
Bootstrap also currently supports all major browsers (including IE) with only a few user experience modifications specific to each browsers quirks. This has enabled me to develop primarily on my browser of choice (Chrome) with the security and peace of mind knowing that it will still function properly on other browsers without requiring me to manually test each browser every time I mess with the CSS or make a change.
So far I very impressed with Bootstrap and I feel that it makes for a great lightweight CSS framework companion. Combine it with jQuery and you have an impressive set of tools for crafting the front-end of your next webapp.
Download Bootstrap from GitHub here: https://github.com/twitter/bootstrap
Find out more about Bootstrap’s history from the horse’s mouth here: https://dev.twitter.com/blog/bootstrap-twitter