Mayhem Gaming: Mayhem Gaming
This is a great website for DM's and players alike. They use OpenRPG, which is specifically meant for dungeons and dragons campaigns online. Dice rolls, character sheets, etc.. are all on it and provide an extremely robust and easy to use system. A lot of people use skype to communicate as well, but some do standard chat. The people are very friendly there and have sub-sections for specific campaigns.
I was surprised that at least OpenRPG isn't in any of the stickies (at least that I could see).
Just thought I'd share my 2 cents
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