I've found a couple that are pretty good:
http://www.homerepairforum.com/forum/plumbing/
http://www.johnbridge.com/vbulletin/showthread.php?p=400559#post400559
The first is for general information, the second is the John Bridge tile forum -- everything you ever wanted to know about tile setting, leak proof showers, etc. Both are vBulletin.
A friend of mine says that Fine Homebuilding has a good one but I think there's a fee involved. If you can find a library that has a collection of those, that's a pretty good resource. Index in the year ending issue, IIRC.