Although Steam's epic summer sale hasn't kicked off yet (perhaps next Thursday?), there are still plenty of deals to be had with 75% or more off Left 4 Dead 2, Killing Floor, Gary's Mod, BioShock titles, Dead Island, the Total War franchise, Spec Ops: The Line, Sleeping Dogs, The Darkness 2 and plenty more, not least of which are Firaxis and Rockstar packs that offer three major titles from each developer for $20. Also of note, this week's Humble Weekly Sale offers savings on Toki Tori, Edge and Rush, while the latest Humble Bundle serves up six indie novels.

Steam
89 Left 4 Dead 2 $4.99 (75% off + free weekend)
77 Killing Floor $3.99 (80% off)
83  X3: Terran Conflict $7.99 (50% off)
Gary's Mod $2.49 (75% off) 
75% off Sonic titles (free weekend for Sonic & All-Stars Racing Transformed)
More...

Amazon Digital
95 BioShock 1 + 2 $4.99 (88% off)
92 BioShock 1 + 2 + 3 $39.99 (60% off)
87 Civ IV, V + XCOM: Enemy Unknown $19.99 (83% off)
88 GTA IV + LA Noire + Max Payne 3 $19.99 (50% off)
88  GTA Complete Pack $19.99 
90 Batman: Arkham Asylum + Arkham City $11.49 (77% off)
80 Scribblenauts Unlimited $7.49 (75% off)
89 Bastion $4.49 (70% off)
More...

GameStop
90 The Witcher 2: Assassins of Kings $7.99 (60% off)
68 SimCity $39.99 (33% off)
65 Defiance $39.99 (33% off)
More...

GamersGate
76 Hearts of Iron 3 Collection $4.99 (75% off)
76 Hearts of Iron Collection $24.99 (75% off)
74 Dead Island: GotY $4.99 (75% off)
60 Dead Island: Riptide $19.98 (50% off)
90 Total War Master Collection $25.00 (75% off)
68  Saints Row 2 $3.74 (75% off)
Adventure Four Pack $9.99 (75% off)
More...

GameFly
94 TES V: Skyrim Legendary Edition $39.99 (33% off)
85 Fallout: New Vegas $4.99 (50% off)
70 Strike Suit Infinity $3.49 (50% off)
Galaxy on Fire 2 Full HD $4.99 (75% off)
Rising Storm $9.99 (50% off)
Kung Fu Strike: The Warrior's Rise $2.49 (75% off)
More...

Green Man Gaming
75 Spec Ops: The Line $7.49 (75% off)
81 Sleeping Dogs Pack $16.23 (75% off)
62 Lost Planet 2 $7.49 (75% off)
88 F.E.A.R. Pack $13.74 (75% off)
44 Stronghold 3 $7.49 (75% off)
Eve: Asteroid Miner Starter Pack $3.99 (80% off)
More...

Games for Windows
77 The Darkness II $7.49 (75% off)
78 Grotesque Tactics Bundle $15.99 (47% off)
65 Painkiller Hell & Damnation Collector's Edition $7.49 (75% off)
56 Duke Nukem Forever $4.99 (75% off)
89  Far Cry 3 $34.99 (30% off)
85  Far Cry 2 $2.49 (75% off)
Far Cry $2.49 (75% off)
Civilization III Complete Edition $2.49 (50% off)
More...

GOG
60% off "amazing stories"