You can get the following games for free from Stem

Free Games for All Steam Users

• America’s Army 3
• BattleForge
• Between
• Codename Gordon
• COIL
• Mightier
• Peggle Extreme
• Sam & Max: Episode 4
• TrackMania Nations Forever

Free Steam MOD Games for Owners of Any Source (HL2) Engine Game with Source SDK

• Age of Chivalry
• D.I.P.R.I.P. Warm Up
• Darkest Hour
• Dystopia
• Eternal Silence
• INSURGENCY
• Mare Nostrum
• Smashball
• Synergy
• Zombie Panic! Source

If you are an AMD ATI (Radeon) user

• Half-Life 2: Deathmatch
• Half-Life 2: Lost Coast

If you are an NVIDIA user

• Half Life 2 – Death Match
• Half life 2 – Lost Coast
• Peggle Extreme

Read more and get the download link from here