Roll20 is a web-based virtual tabletop for pen-and-paper RPGs. Premium features are offered, but at its core this is free, and thanks to its use of HTML5, platform agnostic (thanks Kxmode). Here's word:

Who's this for?
Roll20 is a virtual tabletop. So it's all about taking any game that you traditionally play on a table (especially pen and paper RPGs and board games), and bringing it online. Play with friends who've moved across the country, or find a completely new gaming group online. Roll20 is easy to use, so dive in and get started!

Is this really free?
Yes! Roll20 is absolutely free to use for both GMs and players. Initial development of Roll20 was funded via Kickstarter, and we do offer totally optional supporter accounts for those who want access to additional perks (like more storage space and direct access to the Dev team) to help fund further improvements. But even if you're on a Base account for free, you'll still have access to everything you need to run your game -- unlimited campaigns and players, with access to all the great features listed on this page.