Its been awhile now, but I used to use the
kloogewerks tabletop to run a Conan D20 campaign online. I liked it a lot better than the competitors at the time. Most of those were basically a combined chat, dice rolling, and character sheet program. You can move tokens around a virtual map but all of the heavy lifting is still done by hand. Kloogewerks automates a lot of the calculations like to-hit rolls, spell use, initiative, threatened areas, etc. so it makes life easier on the GM. I know there are both 3E and 4E rules integrated and there is a pretty active group of users who are improving the 4E rules. It is also java so any OS can run it. You can see chatlogs and some screenshots from the campaign I ran
here.
It also has some downsides. It is feature rich so the learning curve can be a bit steeper than other VTs. That didn't bother me because I get into the geeky stuff, but a lot of the players had difficulty with the learning curve especially because the nature of an online game means that the player base is constantly shifting. So I had to do a lot of management of how other people used the system and we would sometimes have to wait for awhile why the newbie figured out how to swing his sword at the goblin. Still all in all, I liked it a lot. I even have a rough draft of a Mechwarrior 3 ruleset for it.
