I have just recently become interested in sumo. You might think that it’s just big men in diapers pushing each other around, but there is much more to it than that. The basic rules of the sport are quite simple (get the other guy out of the ring), but the history and the culture behind it are fascinating.
I don’t purport to be an expert on sumo, so I will just point out some sites that I have found useful.
Sumo mailing list archives.
Lots of stats.
Nihon Sumo Kyokai
The official site for sumo stats and rikishi biographies. Also offers live streaming of bouts.
SC Group Sumo Information
Includes excellent sumo FAQ.
Sumo Fan Magazine
Good explanatory articles.
Sumo Mailing List
List with very high traffic. Definitely subscribe if you are interested in regular updates on all things sumo!