Well for the web developers out there, sitepoint.com has released a really sweet add-on for FireFox that is a reference for HTML elements, attributes, and CSS properties.

It integrates into Firebug which is nice. I must admit that even though I develop in Visual Studio.NET and have all the MSDN documentation, this is a nice reference tool.

Check it out at http://tools.sitepoint.com/firescope