EnhanceJS

EnhanceJS is a new JavaScript framework (a single 2.5kb JavaScript file once minified/gzipped) that that automates a series of browser tests to ensure that advanced CSS and JavaScript features will render properly before they’re loaded to the page, enabling you to build advanced, modern websites without introducing accessibility problems for people on browsers that aren't capable of supporting all advanced features. It's designed to be easily integrated into a standard progressive enhancement workflow: just construct the page using standard, functional HTML, and reference any advanced CSS and JavaScript files using EnhanceJS to ensure they're only delivered to browsers capable of understanding them.

EnhanceJS is written with plain old JavaScript, so it has no dependencies, and works alongside other JavaScript libraries such as jQuery, Prototype and Dojo. The list of browsers that pass the default EnhanceJS test suite includes modern browsers back to Internet Explorer 6, Safari 3, Firefox 1.5, and mobile safari (iPhone); while browsers like Internet Explorer 5.5, Safari 2, and the Blackberry browser will receive the basic experience. The EnhanceJS test suite can be configured to meet the needs of any project, so the division of browsers will change depending on the capabilities you choose to test.

http://www.filamentgroup.com/lab/introducing_enhancejs_smarter_safer_apply_progressive_enhancement/

http://code.google.com/p/enhancejs/