Gametable: The
interface is very simple, but it's easy to use, and it's free. If you're
just looking for an online "gaming mat" this is perfect for you. If I ran
D&D online, this is probably what I would use in combination with Teamspeak.
Not So Free:
Fantasy
Grounds: This is an online "table-top" interface for
D20 gaming. You can read my full write-up here.