Found a cool way to experiment with web development where you can enter an experimental HTML, CSS, and Javascript and show it's output. It's called JSFiddle.
Check it out:
http://jsfiddle.net/user/dashboard/
You do not need to sign up to start experimenting. But to create "fiddles" (i.e., stored experiments), you need to sign up and get an account.
What is good about this service is that, you can import several pre-set JS libraries including jQuery (of different versions), YUI, etc.
Only thing is that you can only choose one library at a time (you cannot import several at a time). But nonetheless, it is good enough when you want to do quick experimentation especially when building on a Java web application which would otherwise require you do a lot of configuration (not to mention the overhead of startup) just to bring up a page.
