He has two main franchises, Avernum and Geneforge. IMO one of the best things about both series' is that he uses descriptive walls of text to describe new areas you enter and the mannerisms of the NPCs you talk to, which helps compensate for the crude graphics and are actually very well written and good at setting mood.
Avernum was heavily inspired by 80s and early 90s computer RPGs, and plays almost like Dungeons and Dragons in that respect. Party of four, large world with lots of quests and secrets and magic items and stuff. This is what your character screen looks like, basically:
So its worth noting that he's remaking the first one into
Avernum: Escape from the Pit, which will be releasing early next year. That'll be on Steam as well, so if that sounds interesting I'd wait for that. The original game looks like this:
The remake will look like this:
http://www.avernum.com/avernum/shots.html
Geneforge on the other hand is inspired more by games like the original Fallout titles, or Baldur's Gate. The first game in the series, just called Geneforge, is pretty much my top RPG of all time. It hits what I think is an incredible balance of exploration, mystery, faction-based politics and more. Read my review here:
http://www.neogaf.com/forum/showpost.php?p=24339218&postcount=336
The other games in the series are definitely solid, although not as amazing. But the first game is something I think every RPG fan should at least play once.