Twitter Bootstrap  

Fresh out of the oven from Mark Otto and Jacob Thornton, Twitter Bootstrap could significantly cut the lead time required when spinning up your next web app. The form elements and notifications are particularly gorgeous.

From Mark’s post on Twitter’s dev blog:

In the earlier days of Twitter, engineers used almost any library they were familiar with to meet front-end requirements. Inconsistencies among the individual applications made it difficult to scale and maintain them. Bootstrap began as an answer to these challenges and quickly accelerated during Twitter’s first Hackweek. By the end of Hackweek, we had reached a stable version that engineers could use across the company.

With the help and feedback of many engineers, Bootstrap has grown significantly to encompass not only basic styles, but more elegant and durable front-end design patterns. This release represents our first public 1.0 release and the open sourcing of many months of hard work.