Modernizr is a handy JavaScript library that detects the capabilities of the browser viewing your site, and then dynamically adds classes to the <html> root element on the page, based on what it detects. You can then write your CSS and jQuery selectors using these classes. An example use would be to detect support for the @font-face CSS property (I am doing that on this site). You can also use it to detect support for various HTML5 elements. I have also heard of people using it to make their sites more mobile browser-friendly.