The user profile for this site ( http://area51.stackexchange.com/users/9999/userName ) is a good tool for keeping track of some of the sites that you have joined up. But this is only incidental in that you have followed that site's proposal in area51 (you can't do that for launched sites).
How about a simple listing of all sites that you have joined up with?
See also, my semi-related, other question:
Unable to follow sites that have started private beta ... where one of the mods, Robert Cartaino, has kindly added the ability to follow sites on area51 after it has entered private beta, which allowed me to keep better tabs on it.