Below is a list of games that are in the Steam content database, that don't appear on the storefront yet. Most of these are games that will be coming to Steam soon. A couple of these games aren't even announced for PC yet (NyxQuest and Death Tank, for instance). A few of these are games that were submitted to Steam a long time ago, but for one reason or another, will probably never go live (A Vampyre Story, MOH Pacific Assault, and NWN2 are all goners, probably). At least one of these is a free mod.

A Vampyre Story
Alien Breed Evolution: Episode 1
Axel and Pixel
Babblix
Batman: Arkham Asylum - Game of the Year Edition
Blacklight: Tango Down
Bloody Good Time (sequel to The Ship, by Outerlight. Includes dedicated server)
Call of Duty: Modern Warfare 2 - Untitled DLC Pack #2
Cesar Milan's Dog Whisperer
Crashday
Death Tank
Defense Grid (Unknown DLC)
Descent: Freespace - The Great War
Dimensity
Dinertown Detective Agency
DogFighter
Elements
Eversion
Faerie Solitaire
Final Fantasy VII
Final Fantasy VIII
Fitness Dash
Freespace 2
Front Mission Evolved (+ untitled DLC)
Garden Defense
Gemini Lost
Gemsweeper
Glyph
Glyph 2
Guns of Icarus
Happy Wars (Fat Princess-style game by Japanese developer Toylogic)
Iron Grip: Warlord
Just Cause 2 - absolutely a shitton of DLC.
Kane & Lunch 2 (plus untitled "DLC1" and "DLC2")
Letter Lab
Lord of the Rings Online
Mafia
Mafia II
Magic: The Gathering - Duels of the Planeswalkers (plus a shitton of DLC)
Mahjong Roadshow
Majesty 2 - lots of DLC
Medal of Honor: Pacific Assault
Moon Base Alpha
Natural Selection II
Neverwinter Nights 2
Neverwinter Nights 2: Mask of the Betrayer
Nightshift Code
Nightshift Legacy
NyxQuest (port from Wiiware)
Overgrowth
Pac-Man
Panzer Killer
Parking Dash
Prisoner of War
Pulse (http://www.pulsethegame.com/)
Puzzle Dimension
Quarrel (http://www.denki.co.uk/games/quarrel/)
Rainbow Six (the original)
Rainbow Six 2: Rogue Spear
Rainbow Six 2: Rogue Spear - Covert Ops
Rainbow Six 2: Rogue Spear -Urban Ops
Rise of Nations Gold
Rocket Knight
SAW 2
Slyder Adventures
Snail Mail
SpongeBob SquarePants: Nighty Nightmares
Stoneloops
Stronghold
Stronghold 2
Stronghold Crusader Extreme
Stronghold Legends
The Legends of Arkanoid
The Saboteur
Trijinx
Unreal Development Kit
Valerie Porter
Valia
Worms Reloaded

Sega Classics (emulated Sega Genesis games, played via a central "Sega Classics" frontend/hub on Steam. Individual games will be sold as "DLC" for this hub. Initial line-up should include Altered Beast, Comix Zone, Crack Down, Ecco the Dolphin, Gain Ground, Golden Axe, Shinobi 3, Sonic 3D Blast, Space Harrier II, and Vectorman)