GET IT NOW.
The hit games that Steam was originally made for is Half–Life and Counter–Strike. It evolved into what is now a portal consisting of game servers, games e–mall and alot of new freebies added on just for those who haven’t already purchased HL or HL2.
If you want to play any of the following games, you’ve gotta buy Half–Life 2 for the CD–Key that will generate your unique SteamID for lifelong use with anything we are allowed to access via steam.
Recommended: Half–Life: 2, jump into the already highly modified world and forget the old shit, also gives you access to Counter–Strike: Source, the most revered FPS MOD online imo.
For hardcore series-lovers: The Valve Premier Pack consists of ALL games prior to the original Half–Life game. Sure bet to keep you on a streak of gaming evolution from 1998 right upto current day. Damn! I can't believe I've been playing HL CS for so long! Check out the entire chronology at Valve.