My profile says November 28, 2003. I have no idea why I created the account back then... I didn't play Counter-Strike, so that wasn't it.
The first entry in my Account Purchase history is from June 2008, when I gifted my dad Portal, but that's obviously a limit to the purchase history rather than an accurate report of my first purchase.
Like others here, the first thing I remember using my Steam account for was buying and playing Half-Life 2.
Was there some kind of update to the first Half-Life and/or its expansions if you registered your game with Steam? I have some vague memory of something like that, but I could be wrong.