Tabletopia is a simulated boardgame system, playable solo or multiplayer through a browser or via Steam. With over 500 games currently playable, it's a great way to try out games which aren't yet released, aren't readily available, or are costly. There are occasional glitches but's well worth checking out not least because most of the games are free.
So far, the best game I've tried is the innovative Roll Player, designed by Keith Matejka and published by Wisconsin-based Thunderworks Games. It's a dice puzzle game for 1 to 5 players which is closely based on the character creation process in RPGs like Dungeons & Dragons - and while that might not sound fun, Matejka has done an incredible job.