One style of game that seems to work great under the OSS model is roguelike games. Granted, they tend to have cheesey graphics (or no graphics at all), but I've found the replay and entertainment values for Zangband, NetHack or the others orders of magnitude greater than, say, Diablo.

It's also interesting that code-forking seems to be quite a healthy thing for these games. There are scads of variants (esp. off of the Angband code-base) and each has interesting game quirks that authors have added.

They may not be commercial quality, but they're often more fun that commercial CRPGs.

-Dana