Is there anything that keeps GOG from being a sort-of "freeware" aggregation system? There are a bunch of older games that have been released as freeware that publishers have forsaken but have not released the rights to, but if I knew I could find them on GOG, I'd be more inclined to browse here, even if I'm grabbing a freeware game now and coming back to buy Perimeter, for example, later.
There are some quality games that have been released that only call FilePlanet, a lone torrent, or other out of the way websites their home. The pursuit of publishing rights or if you would seek to put a price tag on them, well, that would be totally up to you, but it's certainly a possibility. Most of these older games, being released as freeware, don't have any DRM to speak of, so getting them to work on Vista / XP would be the biggest problem.
If you'd like some examples, I'd certainly present Ground Control, MechCommander 2, and Starsiege: Tribes. I still play these games every now and then; Tribes is one of the best multiplayer experiences to date, Ground Control was a fantastic RTT game (much the predecessor of the excellent "World In Conflict"), and MechCommander 2's gameplay is different then most.