This is simply an amazing value..

Valve Complete Pack

Games included:

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
Peggle Extreme
Portal
Ricochet
Team Fortress 2
Team Fortress Classic

Also...Don't forget all the modded games this will give you for free:

Action Half-Life
Adrenaline Gamer
Age of Chivalry
Alien Swarm: InfestedAwakening
Battle Grounds 2 (beta)
BrainBread
D.I.P.R.I.P. Warm Up
Digital PaintballDystopia
Earth's Special Forces
Empires Mod
Eternal Silence
Fortress Forever
Frontline Force
Garry's MOD 9
Half-Life 2: Capture the Flag
Hidden: Source
Insects Infestation
INSURGENCY: Modern Infantry Combat
International Online Soccer
MINERVA
Natural Selection
Pirates, Vikings and Knights II
Plan of Attack
Revolt The Decimation
Riot Act
Rock 24
Science and Industry
Source Forts
Sven Co-Op
Synergy
The Battle Grounds
The Specialists
The Trenches
Vampire Slayer
Zombie Panic
Zombie Panic! Source

I'm pretty sure all these are playable if you have the HL1 & HL2 engine.