What game should I get my 12 year old brother for Christmas from steam?

DarkSiders is a good call and it's a daily for a few hours yet. I played(and highly enjoyed) Ocarina of time around the same age and the game concept's pretty similar. Without further knowing the tastes of the kid it's pretty hard to suggest something though. Batman Arkham series is also highly awesome and comic book related to boot(if he reads any that is.). If he likes medieval combat and RPG I'd suggest any Elder Scrolls and the aforementioned chivalry/War of the Roses, I particularly like WOTR more but Chivalry has more players and is more fast paced.

I enjoyed Return to Castle Wolfenstein when I was 12. Good old times...

CS or Dota 2.

Postal 2 :3 trollface

Diablo 1. Except it's not on Steam but it owns.

Fortix, BIT.TRIP RUNNER, any indie game :P

scribblenauts unlimited or whatever. The newest one.

Vice City

Almost any driving game or sports game. My son LOVES the Dirt series.

Buy him a NES and get him anything from the Megaman series or Ghosts 'n Ghoblins.

Tiny Bang Story, Eufloria, The wonderful end of the world, rayman

Bastion, Limbo, Super Meat Boy, Lego Batman, Torchlight, Terraria, and Magicka.

Bastion; Quantum conundrum; Portal (1 and/or 2); Dungeon Defenders. Mainly strategic and 'group '''friendly'''' type of games. Torchlight 2 and Spores are well. (get creative?)

Terraria
or wait for Starbound

terreria?

Psychonauts

Two Worlds II?

Spore
Any of the doom games except Doom 3 if your ok with slightly violent games.
Hexen and Heretic which use the same engine as doom but are more fantasy based.
Magicka
Any of the Sonic games.
Dungeons of Dredmor
Commander Keen
Audio Surf if he has a decent music collection.
World of Goo.

Rayman Origins for sure! Sonic Generations is also pretty good.

This isn't a steam game, but if he is into RPG's I'd suggest checking out www.wakfu.com/

Audiosurf

and FORTIX

If u never wanne see him again get him Torchlight II

A link to profile?