Currently I am running a Babylon 5 game using the Hero system. Because I'm using someone else's world, I'm uneasy about making up things that are blatantly contradictory to canon, at least when there are so many resources available on the Web. Here are just a few of the ones I'm drawing from.
Lurker's Guide includes lots of info, but the best part is the comprehensive episode list.
B5 Tech Manual trying to mesh hard science and B5. I don't know how much is canon, but this is one of my main sources for the game.
"Unofficial B5 Technical Manual" by By Thomas Chiverton
Hyperspace: A Guide to B-5/Crusade Ships
Time line of the B5 universe
Transfer Point Io, featuring dozens of B5-related links (many of which are no longer active, unfortunately).
Here is a hex map of known space, with territories, created for a play-by-e-mail game. Requires Adobe Acrobat to read.
This is a nice star map except for the fact that it is based on a timeline where Earth lost the Minbari War.
Planets of the Earth Alliance: compiles all colonies mentioned in the B5 world.
Detailed information of the Babylon 5 Station, found on the Babylon 5 MUX web page. I don't know how much is canon.
You can find the B5 Hero System rules set we are using at this site. Feel free to comment.
Here you can find B5 fiction from JMS and regular B5 writers, as well as Crusade scripts which were never filmed.
Wolf's Shipyard presents scale drawings of B5 ships, both canon, game and fan designs.
Other B5 Sites
The B5 that Never Was is a page outlining some of the changes that occurred to the B5 storyline because of cast changes etc. Not useful from a gaming standpoint, but interesting for a fan of the show.
Art of the Technomage: A site demonstrating how close we can get to technomagic using existing technologies. This group was inspired by B5 characters, but these aren't fanpages.