Not a GOG, but Super Meat Boy should work nicely. The individual levels are short but very hard. You will die countless times during a 20 minute break. SMB's hardware requirements are also pretty low.

Got Abe's Exoddus? Abe's Oddysee is good too, but no quicksave if you want to be able to save and quit/

Wow, didn't expect so many replies, I will sort through them during my next break, but I can already see a few suggestions that seem great.

I'll let you know what worked and what didn't :)

Thanks a lot all!

+1 for Puzzle Quest

Blood
Duke Nukem 3D
Earthworm Jim 1+2
Guilty Gear X2
Pod Gold
Raptor: Call of the Shadows
Rayman Beyond

Okay, I've sorted it out a bit, here's what I have so far:

Good suggestions but already played:
Limbo
Osmos
HOMM (all of them)
Audiosurf

Currently testing:
Oddworld games
Duke Nukem 3D
Super Meat Boy

Will definitely try but hadn't thought of them:
Master of Orion fames
Lord of the Realms 2 (have played this one ages ago, fond memories)
Max Payne
Nox
Plants vs Zombies

Not really my cup of tea, will probably pass (but thanks anyway!)
World of Tanks
Puzzle Quest

Have no idea what that is need to check it out:
Creeper World
King of Dragon Pass
All the cool stuff XYCat suggested :-)
The last post from Ric1987 :-)

Thanks again!

oh my god the Super Amazing Wagon Adventure is awesome!!! reminds me of Vidiot Game a little ;)

Some people have rightfully suggested Puzzle Quest, so I would say "Sequence" is a good bet too, just replace the bejeweled gameplay with a rhythm game gameplay and there you go.

Frozen Synapse is really good too as there's a configurable skirmish mode.

Dungeons Of Dredmor is a good option too (you can turn off the perma-death if you want).

For quick, short bursts, I recommend Realm of the Mad God. It's a rogue-like shooter in a fantasy setting.

Puzzle Quest and Bejelewed 3

Faerie Solitaire. Great simple game.

City of Heroes for the next two months while it's alive. It's a great MMO for playing short sessions.

FTL seems tailor made for short playsessions. Also, I should mention that my own game was created for this specific purpose (and it's free).

http://www.indievania.com/games/pit-bite-sized-shooter