What are some alternatives?
When comparing Roll20 and Astral Tabletop, you can also consider the following products
Fantasy Grounds - Fantasy Grounds is a virtual tabletop to help facilitate play of tabletop-style role-playing games...
Beyond Tabletop - Beyond Tabletop is a set of tools for RPG players with easy-to-use online character sheets and maps.
GM Forge - GM Forge is a Standalone Virtual Tabletop and is a sandbox for GMs.
Rolisteam - Rolisteam helps you to manage tabletop role playing games with remote friends/players.
MapTool - MapTool is an online, multiuser, networked, graphical, interactive, programmable virtual tabletop.
Dungeon Painter - Dungeon Painter is a free powerful encounter map design tool.