I got a lot of them free with other games during the steam sales.

go here http://www.steamcalculator.com/id/
Code:
Account Information
Username: Gangsta Omlett
CommunityID: 76561198033584580
Custom URL: omlett
SteamID: STEAM_0:0:36659426
Status: Online
Steam member since: 2010-11-14
Found 50 Games with a value of
$655.51 USD

Game Title	Price	Total Running
01	18 Wheels of Steel American Long Haul	$9.99 USD	$9.99 USD
02	18 Wheels of Steel: Extreme Trucker	$19.99 USD	$29.98 USD
03	Amnesia: The Dark Descent	$19.99 USD	$49.97 USD
04	And Yet It Moves	$9.99 USD	$59.96 USD
05	Atom Zombie Smasher	$9.99 USD	$69.95 USD
06	Cogs	$9.99 USD	$79.94 USD
07	Counter-Strike: Source	$19.99 USD	$99.93 USD
08	Crayon Physics Deluxe	$19.99 USD	$119.92 USD
09	Darwinia	$9.99 USD	$129.91 USD
10	DEFCON	$9.99 USD	$139.90 USD
11	DiRT 3	$49.99 USD	$189.89 USD
12	Eufloria	$14.99 USD	$204.88 USD
13	Far Cry®	$9.99 USD	$214.87 USD
14	Far Cry® 2: Fortune's Edition	$19.99 USD	$234.86 USD
15	FlatOut 2™	$9.99 USD	$244.85 USD
16	FlatOut: Ultimate Carnage	$19.99 USD	$264.84 USD
17	Garry's Mod	$9.99 USD	$274.83 USD
18	Half-Life	$9.99 USD	$284.82 USD
19	Half-Life 2	$9.99 USD	$294.81 USD
20	Half-Life 2: Deathmatch	$4.99 USD	$299.80 USD
21	Half-Life 2: Episode One	$7.99 USD	$307.79 USD
22	Half-Life 2: Episode Two	$7.99 USD	$315.78 USD
23	Half-Life 2: Lost Coast	$0.00 USD	$315.78 USD
24	Half-Life Deathmatch: Source	$9.99 USD	$325.77 USD
25	Half-Life: Blue Shift	$4.99 USD	$330.76 USD
26	Half-Life: Opposing Force	$4.99 USD	$335.75 USD
27	Half-Life: Source	$9.99 USD	$345.74 USD
28	Hammerfight	$9.99 USD	$355.73 USD
29	Hitman 2: Silent Assassin	$9.99 USD	$365.72 USD
30	Hitman: Blood Money	$9.99 USD	$375.71 USD
31	Hitman: Codename 47	$9.99 USD	$385.70 USD
32	Hunting Unlimited 2010	$9.99 USD	$395.69 USD
33	Hunting Unlimited™ 2008	$9.99 USD	$405.68 USD
34	John Deere Drive Green	$29.99 USD	$435.67 USD
35	Midnight Outlaw: 6 Hours to SunUp	$9.99 USD	$445.66 USD
36	Plain Sight	$4.99 USD	$450.65 USD
37	Poker Night at the Inventory	$4.99 USD	$455.64 USD
38	Portal	$9.99 USD	$465.63 USD
39	Portal 2	$29.99 USD	$495.62 USD
40	Prison Tycoon 3™: Lockdown	$9.99 USD	$505.61 USD
41	Prison Tycoon 4: SuperMax	$29.99 USD	$535.60 USD
42	Ride! Carnival Tycoon	$9.99 USD	$545.59 USD
43	S.T.A.L.K.E.R.: Call of Pripyat	$14.99 USD	$560.58 USD
44	S.T.A.L.K.E.R.: Shadow of Chernobyl	$19.99 USD	$580.57 USD
45	Sniper: Ghost Warrior	$19.99 USD	$600.56 USD
46	Sprint Cars Road to Knoxville	$9.99 USD	$610.55 USD
47	Steel Storm: Burning Retribution	$4.99 USD	$615.54 USD
48	Team Fortress Classic	$4.99 USD	$620.53 USD
49	Trackmania United Forever Star Edition	$29.99 USD	$650.52 USD
50	VVVVVV	$4.99 USD	$655.51 USD