What was your first game on steam? GA ended.

Counter Strike 1.6 :)

Audiosurf, came up with a steam copy bought of their site, redeemed it on steam but never used steam for the first 6 years.

My first Game on Steam was Deponia and it is not long ago, i am since 3 years on Steam and now have 360 Games ;)

Counter-Strike 1.6

Portal

Anomaly Warzone Earth.

TF2+Garries mod bundle.

Left 4 Dead 2 for $5, I thought that was super cheap. :)

I can answer in a second this. Serious Sam TFE HD! Bought the DVD and expecting to play off DVD as usual and perplexed it wouldn't work till I read the fine print and discovered I needed to install a strange software called Steam I had not heard of before and play online.

Well if the beta of Dota 2 counts, that mine. It's tied to my account and doesn't dissapear after deleting it from library.
The first ever buyed game however, is Terraria! (Which I buyed on full price because I didn't know much about Steam). One later it got on 2 euros discount.
Also the first free game I got from a GA was Pid!

LYNE + Pixel Puzzles: Japan

Dawn of War: Soulstorm

Originally my brother and I shared a steam account as soon as it came out, first game was probably HL2 or Orange box.....

I eventually made a new account and my first game was Alien Swarm so I could Co-op with my bro, I still cannot believe that game is free....so awesome.

Deus Ex: Human Revolution - bought it on premiere and back then I was quite angry about it requiring Steam - I bought a box copy on Day 1 and you expect me to download and install some shitty DRM Client and at top of that download game anyway instead of installing it from Disc? Outrageous! :D: For some time I didn't get invested into Steam (back then I was still sinking 99% of my time into WoW only, playing maybe 2-3 other games per year), then I won Skyrim in some FB competition on premiere as well, it also required Steam so I remembered I have account already. And then while I was still playing Skyrim so still loging into Steam Winter Sale came... ;p

The fruit of grisaia, been waiting months for the release

Counter Strike: Source

CS 1.6 obviously :)

Portal 1, sneaky Valve, gave me a free game and almost 1000 games later Im still buying.

Amnesia: The Dark Descent.

Dota 2 as a free game, my first purchase was Humble Origin Bundle which was a great bundle

Half-life 2 along with Counter-Strike:Source

According to my Steam License History, Half-Life Platinum Pack - Activated 4 September 2004.
I seem to recall this was because some time around then for CS 1.5 they removed the ability to download updates for CS from a website, and required Steam for updates (and my copy of CS 1.5 came in the Half-Life Platinum Pack), but it was 12 years ago, so my memory may be wrong.

The Witcher 2

Non-paid = TF2
Paid = Call of Duty: Ghosts Pre-order Edition

30 Nov, 2010 NVIDIA Portal: The First Slice Complimentary
This one 1st but was free
21 Dec, 2010 Toki Tori Steam Store
21 Dec, 2010 Left 4 Dead 2 Steam Store
21 Dec, 2010 Counter-Strike Complete Steam Store
^After that I purchased those all in 1 go so can't say specificaly which one is 1st :)