Batman: Arkham Asylum, that came with an nVidia card. I remember installing Steam (which at the moment I knew nothing about), redeeming the key, and then not even touching Steam for another two years :)

I've used a few Steam accounts,but I think the first one I ever bought was Counter-Strike Anthology

Counter Strike: Source.

HF2 collectors in a box with a shirt

Left 4 Dead 2 was on sale by 75% :D

ATI Half-Life 2: Deathmatch / Half-Life 2: Lost Coast Promo.

First L4D

Torchlight 2 for 15 dollars preorder and I REGRET IT!

The Orange Box, which I originally bought for Portal, and Portal 2 (both retail).
That's what got me to try HL and TF and got me into shooters (and generally got me far deeper into gaming :P)
I absolutely hated Steam at first XD

Cs 1.6 and still remain the best game for me.

Grand Theft Auto: Complete pack - 14 €

Team Fortress 2, because it was free (August 2011). The first game that wasn't free was the Penumbra: Black Plague Gold Edition. I needed to buy a game to have friends, and it was survival horror.

Half-Life Platinum Pack

my first Steam game was Supreme Commander 2 back in November 2010

Napoleon: Total War

Dead Island :3

Counter-Strike

I bought PVZ first -_-

FEAR 2: Project Origin

IIRC, Codename Gordon

Portal 2 when it released.

I bought my first game on steam about 4 months ago.. March 7th 2013 to be exact. it was Spectraball. I bought it on sale for $1.24. I now have over 200 games that I've purchased (plus several f2p's). I never thought I'd say this, but Steam has gotten me to stop pirating.

Team Fortress2, next was premium update and story has begun...

Half life/Counterstrike

Orange box, but the first game I played from it was portal