I bought Half-Life 2 at release (November 17, 2004.) The content servers were straining quite hard that day but I had a good connection and managed to get in without too much delay.

Dota 2

Free Portal, but I couldn't play it at the time. I kept the account til I got a better computer, and fittingly enough, Humble Bundles started about then, and I started to notice great sales and I've stayed since. Kinda.

Got free Magicka and then Cities in Motion :)

Because Left 4 Dead (2) required Steam.

I didn't join steam, steam joined me.

bought l4d and welp, had to install it

Free portal did it for me too.

The prices during a Christmas sale. When I saw how cheap they were, I finally caved, after avoiding them like the plague since the Half Life days. A lot of gamers despised Steam at first, including me, They've gotten much better since then so I don't mind these days.

Free portal and hl2 deathmatch xD

Skyrim said it required Steam :p

Free games when I purchased many video cards.

To activate Half-Life and later Half-Life 2.

My friend told me Borderlands GOTY was on sale on it and that I should get it, so I did. Great game.

to buy games

to install half life 2 when it came out :D

Because i was looking for a great game witch decent social aspect after i stopped playing WoW. Still cant find a game that comes anywhere close.

To play Alien Swarm.

To play civ 5

Why I joined Steam? Valve shut down WON. I had no choice.

Orange Box '07. yayeah!

It was Terraria and Portal for me.

I joined steam when Team Fortress 2 went f2p, but then I found out that it's not my kind of game, so I left steam till I got Borderlands 2 with my graphics card.

To play Serious Sam HD.

For playing CS 1.6