What's your favorite FPS game that many people don't know about?

EDIT: Wow! Cannot believe the response this topic got, here's a list of suggestions:

• 007 Goldeneye
• Area 51
• Blockstorm
• Bulletstorm
• Call of Cthulhu: Dark Corners of the Earth
• Call of Juarez 4: Gunslinger
• Clive Barker's Undying
• Descent 1
• Descent 2
• Dino D-Day
• Forsaken
• Hard Reset
• Insurgency
• Malice for Quake
• Outlaws
• Painkiller Black Edition
• Pokemon SNAP (Game seriously kicks ass though)
• Prey
• RAGE
• Red Orchestra 2 / Rising Storm
• Sanctum
• Sanctum 2
• Shadowcaster
• Shogo: Mobile Armor Division
• Singularity
• Spec-Ops: The Line
• Strife: Quest for the Sigil
• The Darkness 2
• Timeshift
• Timesplitters 2
• Unreal Tournament 2004
• Vietcong
• Wolfenstein (2009)
• Wolfenstein: Enemy Territory
• Ziggurat