Andy Lange has done an excellent job of keeping up with the games that are going on, and has also set-up an opponents wanted list to help those who would like to set up (or join) a game.
Rob Stimets has taken on the task of running a three 1856 games, as well as participating in a few others. He has set up an impressive page for tracking his games, and there is stuff that is interesting for 18xx gamers in general. (Take a look at the Escher train-game...It's hilarious!)
Another site with a game that is just starting up.
The River City Gazette seems to have several 1830 games going on, and has some interesting proposals for standardizing PBW (play by Web) rules.
Dave Mitton has recently put some of his 1830 games on-line.
Another 18xx game site, primarily for 1835.
In addition, there is a game tracked here. A 1835 game, ALPHA, moderated by Bill Stoll, and all of the tools necessary to run a game, can be found off of the 1835 page More games to come - time permitting!
All of this activity begs the question about PBEM ladders. It has been suggested in at least one game I'm involved in that we start tracking people's records. I'd be happy to put such a feature here, but I would like to hear some ideas on how it will be done: specifically, how the rankings will be done (net worth at end of game, finish position, are the number of players factored in, does game length make a difference), and whether there should be listings for different 18xx titles or should all scores be lumped together (or maybe both?)?
If you know of a web-site that supports a PBEM game that is not listed here, please let us know.