Steam has a pretty good deal on a package of games including the upcoming release, Left 4 Dead.
The Valve Complete Pack is priced at $100 and includes:

Counter-Strike
Counter-Strike: Condition Zero
Counter-Strike: Source
Day of Defeat
Day of Defeat: Source
Deathmatch Classic
Half-Life
Half-Life 2
Half-Life 2: Deathmatch
Half-Life 2: Episode One
Half-Life 2: Episode Two
Half-Life 2: Lost Coast
Half-Life Deathmatch: Source
Half-Life: Blue Shift
Half-Life: Opposing Force
Half-Life: Source
Left 4 Dead (not yet released)
Peggle Extreme
Portal
Ricochet
Team Fortress 2
Team Fortress Classic

Might be worth picking up, especially if you haven't played some of the older titles, and are planning on buying Left 4 Dead.