To get us started:

Road of the Dead:
http://www.newgrounds.com/portal/view/550714

Gearlock:
http://www.newgrounds.com/portal/view/549022

Obbey the game!
http://armorgames.com/play/4987/obey-the-game

Machinarium. Probably not what you're looking for in this thread. But it's certainly my favourite all the same. That said, the first Samorost is available for free.

Amorphous Plus

http://armorgames.com/play/2044/amorphous-plus

Addictive.

Reminds me, i have a tape of Erasure. I was a kid, and i thought a guy in a blue fishnet suit is expressing his artism, and a heart cut out on the ass a message of love, like Jesus, and all... I was 7 or 8..

[edit: so many mistakes]

Monster's Den...highly addictive & fun as hell!!

Realm of the mad god.

Ching Chong Beautiful.
I love this game, it's fun to go back and improve on my fastest time in each level.

The Organ Trail.

And no, that's not a typo. Relive your Friday computer lab afternoons from Elementary School with RAVENOUS ZOMBIE HORDES!

Minions by far (I enjoy multi player activities that involve a bit of strategy and action)

The Last Stand 2- Zombie survival game
Mud and Blood 2 - Tactical strategy (in fact not-so-tower-defense) from WW2

Ooooooooooooooooh Pandemic 2... You damn Madagascar !