The best web resource for Babylon 5 information, maintained by Steven Grimm at http://www.midwinter.com/lurk. Grimm's site is well-designed and comprehensive, with detailed episode-by-episode analyses and factoids. Although the site is not updated as frequently due to the series' end, it is still a worthwhile source -- particularly since the Sci-Fi Channel will be picking up the series again this September.
The Guide and its associated FTP archive used to be located at hyperion.com before Grimm and company lost the domain. The EAS Hyperion cruiser in the first-season episode "A Voice in the Wilderness" was named after the site.