I first learned about cellular automata in high school, from the Mathematical Games column in some back issues of Scientific American, talking about John Horton Conway's "Game of Life". I found it fascinating, and even tried some patterns by hand (which quickly takes all the fun out of it).
The Conway's Game of Life page has a Java applet you can use to check out patterns.
Here is the "Mirek's Cellebration" page (here's a mirror site if the first site doesn't work). It has a freeware program and a (limited functionality) Java applet.
Here is the Color Game of Life Visual Exhibition (great for exploring).
Here are the Google Directory, the Yahoo! Directory, and the Open Directory Project's Directory for more information.