Roll 20 includes an integrated text chat but also supports voice and video chat as well as Google Hangout support. There is wide spread support for many systems but the most supported system is 5th edition Dungeons and Dragons.
To get setup to play in a Virtually Expo game via Roll 20 you need to:
- Setup a free account here
- Login to Roll 20 on your browser.
- Follow the link your GM sends you.
For a walk through of the basics of Roll 20 watch this video: