Roll20 - Roll20 is a suite of easy-to-use digital tools that expand pen-and-paper gameplay.
MapTool - MapTool is an online, multiuser, networked, graphical, interactive, programmable virtual tabletop.
Astral Tabletop - Create, host, and manage your tabletop role-playing games — online or in person
Beyond Tabletop - Beyond Tabletop is a set of tools for RPG players with easy-to-use online character sheets and maps.
Rolisteam - Rolisteam helps you to manage tabletop role playing games with remote friends/players.
GM Forge - GM Forge is a Standalone Virtual Tabletop and is a sandbox for GMs.