So I've got a couple of extra TF2 keys

Magicka is a fun game, but its bundled, so worth less than that.

How about Dino D-Day?

Democracy 3?

Batman games? Idk. most games I'm looking at are at least 2 TF2 keys. Sonic & All stars racing, Tabletop sim, HL Complete

Project zomboid

Try looking for some Torchlight 2 or Terraria, ppl were selling Torchlight 2 for 1-2 keys and Terraria for 1key each. Both games are good,have mods, and have co-op as well as lan.

Terraria...

paradox deluxe edition you can find it for 2-3 keys

Tropico 4 Collectors Bundle? :3

I just saw for that price : Bully, Brothers a tale of 2 sons, Cabela's Big Game Hunter Pro hunts (I really enjoyed this one) and Lego the Hobbit

The Escapists, Bastion or Don't Starve Reign of Giants?

Bastion perhaps? Great reviews, above 1 key but still pretty cheap overall (goes for around 1.5keys IIRC)

Lowest key value, hmm? I suggest Chicken Shoot Gold or Heli Heroes.

Bunny Must die? (No idea how many keys that goes for)

Rise of the Triad? It was for 1 key on steamtrades.

Half Life 1 or CS 1.6? :D

FF III,IV,VI,VIII or XIII

Terraria