gotAPI is a cool Ajaxy web app that gives you quick access to reference materials for a variety of web technologies. There's a list of languages including HTML, JavaScript, CSS, PHP, Java, Perl, and six others, and when you click on one of them the list of HTML tags, JavaScript objects, PHP functions, etc. pops up. There's a search box at the top for finding items quickly, and clicking on an item brings up a reference page at the W3C , MSDN, W3Schools, and the like. I wish there were more languages available (Ruby, Python, and XUL, for example), but it's a great one-stop resource of a lot of tasks.