I mention the one here in Cleveland from time to time. They play some of the games at League Park, and use 1865 rules.. Haven't been to a game yet, but I bet it would be a good time. This year's schedule isn't up yet, but the website's pretty cool if you poke around a bit. Gotta bunch of goodies in regards to the history of the stadium, and a bunch of stuff about 19th century baseball in Cleveland.
http://www.clevelandbluesbaseball.com/