I love places like W3Schools that let you play around with HTML and such right there, without you having to mess around with your own text files and servers.

I found one that may be even better than W3Schools, however: HTML Playground. They define a tag, show example code and then show what the code does, which isn’t too special, but they do show it all on one page, in iframe boxes. This is nifty. Go check it out and see for yourself.