Off the top of my head, these are some games that to my knowledge support local co-op:
- Rayman Origins
- Trine Enchanted Edition
- Trine 2
I suspect (but don't know) that these might as well:
- Adventures of Shuggy
- Crawl (upcoming since forever)
- Fractured Soul
- Pixeljunk Shooter
I agree that it would be useful if GOG made a distinction between local co-op and 'more than one PC required' co-op (not sure what would be the exact term).
Local Co-op would probaly be the right term, thanks for the list
The Adventures of Shuggy
The Chaos Engine
Double Dragon Trilogy
Guacamelee (Both editions)
LEGO Harry Potter: Years 1-4
LEGO Harry Potter: Years 5-7
Octodad: Dadliest Catch
PixelJunk Monsters Ultimate
Risk of Rain
Serious Sam: The First Encounter
Serious Sam: The Second Encounter
Tom Clancy's Rainbow Six
Vertical Drop Heroes HD
Supposedly, Age of Wonders 3 and Ground Control II also have a local co-op mode, though I have no idea how it works.
I think it stems from consoles, where you would be able to sit on a couch and play co-op on the large/r TV screen as opposed to a PC monitor.
The PC hooks up to TV screens as well, consoles are becoming obsolete now.