Freespace 2.
It's the most epic space sim I've ever played and has great graphics with the mediavps (source code project)
Fs2 is also how I found out about gog.com lol.

Edit: I'm actually going on a fs2 marathon tonight XD

Kotor 1&2 or Two worlds 2

Baldur's Gate 2.

Why? Because if you really play it from start to finish in a matter of hours, you'll end up in the Guinness Book of Records.

i hear yah

If you're doing a marathon, do Marathon. No it's not a GOG, yet. It's free though (and open-source no less).

Did you read my mind?

Possibly. I was a mac user long before I was a Windows user however, so I like referencing games primarily built for the former platform (and ported to the latter where applicable).

MARATHON SUX DOOM FOREVER WOOOOO!

*Ahem* Just kidding around :P

Arcanum, because it's that good.

In the shorter game department that I know of:

Freedom Force vs The 3rd Reich (been years, but I vaguely recall it being short)
Master of Magic on impossible with max # of players (if you're good)
Master of Orion 1+2 on impossible with max # of players (if you're good, try a smaller galaxy size without creative for an extra challenge)
MDK
Megarace (if you're good)
Moto Race 1+2 (if you're good)
Painkiller
Serious Sam (if you save often)
Raptor: Call of the Shadows 2010 Edition (if you're good)
Another World: 15th Anniversary Edition
Simon the Sorcerer (if you're sharp)

That's what I was about to say...and if you're still craving more, you can go through Marathon: EVIL, and Marathon: Tempus Irae, two total-conversion mods which are meaty enough to be full stand-alone games in the own right.

And you should also be sure to play the current king of Marathon TC mods, the unofficial "Marathon 4", Marathon Rubicon.