Big Dogs does an "official" century-a-month challenge ... that's where I've recorded all mine.
http://www.big-dogs.org/scripts/bdhome.asp
The UMCA has a challenge as well, although I don't like the fact that their centuries can be 90 miles.
http://www.ultracycling.com/
And this forum, and Bicycling Mag's forum, and the Cycling Plus forum, and the Bikejournal forum all have century-a-month challenges too ... if there are other cycling-related forums out there I don't know about, they probably have a similar challenge too.