Here are a list of free, offline, relatively low system requirement games I have.
[Demo, but still awesome] Kerbal Space Program - Build rockets, blow them up, fly to the moon and such.
Battle for Wesnoth - Neat medieval-fantasy strategy game, leveling up is fun.
Stranded 2 - Survive on an island, and go through a really wacky entertaining storyline.
Scorched 3D - Like worms/tanks, turn-based projectile firing map destroyer.
Spelunky - 2D cave explorer platformer, little tough at first.
SilverKnights - 3D robot fighter.
Powder Toy - Literally, a sandbox, with explosives.
Knytt Stories - Great platformer adventure, sucks when you get lost though.
Mari0 - Portal spin-off of the classic Mario series.
Focus - A super addicting puzzle/action game involving slow-motion rocket dodging.
Against the Wall - Kinda pushing the system requirements, puzzle exploration up a wall.
Cave Story - If you don't have this, shame on you.
[Any emulators that are from your childhood, those are good fun]
Halo 1 - Classic 3D alien shooter, very fun. [Might not be free?]
Toribash - Turn-based fighting game where you control joints, really neat.
Blackshades - Simple bullet-time protect the VIP game.
BrickForce - Blocky shooter.
[Demo] Lugaru - Addicting kung-fu rabbit fighting game.
...I think that's all I have...
