A relatively new VTT contender, Astral Tabletop has now partnered with DriveThruRPG:
http://www.enworld.org/forum/content.php?6219-Astral-TableTop-Gets-RevampedSteve Weick, DTRPG's head honcho, posted on Facebook:
We're formally announcing today that DriveThruRPG is a strategic/investment partner in Astral Tabletop, a new service for virtual/in-person RPG tabletop play.
Similar to Roll20, Astral is free (freemium) and web-based, which makes connecting with friends and playing a game on the platform super-easy. Astral's focus is on ease-of-use. It was Astral founder Tom Lackemann's dedication to making the tool simple to learn and use that attracted me to partnering with Astral. It's taking the learning curve out of using virtual tabletops.
It also takes in-browser graphic performance to a new level. If you haven't tried a VTT with features like fully animated maps and weather or localized sound effects, I'd encourage you to give Astral a try and see how immersive it can make your next game.