So, which games in Steam has item drop?

Battleblock Theater

warframe, spiral knights?

edit: most of these games should have drops (i think path of exile doesn't have any though).

Unturned

Minimum, Path of Exile, Altitude0: Lower & Faster, Primal Carnage: Extinction, Ratz Instagib 2.0, Robot Roller Derby Disco Dodgeball, Sins of a Dark Age, Subanutica, The Mighty Quest for Epic Loot

Go to market, then you see all.

There's no item drop in Warframe (if you mean items in Steam inventory).

Played ARK Survival Evolved back when it was in free weekend, and it added an item category in my inventory. I have no idea what kind of item you can receive in there, or even if you can drop items in it.

Many more games since this post was made has added/removed inventory items to Steam, to keep up with them I have made a guide on Steam.
http://steamcommunity.com/sharedfiles/filedetails/?id=873140323

http://steamcommunity.com/market/
almost all these

Don't Starve Together
http://store.steampowered.com/app/322330/Dont_Starve_Together/

PLAYERUNKNOWNBATTLEGROUNDS
Day of infamy

Killing Floor 2

http://store.steampowered.com/app/274940/Depth/