For those who somehow don't know, it's going on right now. Some quick pricepoints for people.
*NOTE: There are additional price cuts called "Flash Sales" going as well, they last 6 hours each for 4 titles at a time, it seems it bumps the discount up to 75%.
*NOTE2: There are also additional price cuts for "Community picks" which are user-voted sales that last 8 hours.
So far we've had the following Community Pick Sales:
1) Saints Row: The Third (-75%, $12.49)
2) Trine 2 (-75%, $3.75)
NOW FOR THE GOOD PART! (This isn't a comprehensive list, almost everything on Steam is at least 25% off right now. These are just a bunch of examples.) Note that DLC are all also discounted.
Portal 2 (-75%, $5.00)
Total War: Shogun 2 (-75%, $7.49)
Legend of Grimrock (-60%, $5.99)
Terraria (-75%, $2.49)
Call of Duty: Modern Warfare 3 (-50%, $29.99)
The Elder Scrolls V: Skyrim (-33%, $39.99)
The Elder Scrolls IV: Oblivion GOTY Deluxe (-40%, $14.99)
The Elder Scrolls III: Morrowind GOTY (-40%. $11.99)
Fallout: New Vegas (-40%, $11.99)
Fallout 3 GOTY (-40%, $11.99)
Ys Origin (-40%, $11.99)
Ys The Oath in Felghana (-66%, $5.09)
BioShock (-50%, $9.99)
BioShock 2 (-50%, $9.99)
Most Indie Titles (-33%~-75%, $1.00~$5.00)
And many, many, many more. Just a heads up, awesome time to save a ton of monies. It's generally good Steam Sale etiquette to wait and see if the game(s) you want go on sale further for daily deals/flash deals since everything stays on sale until the very end of the sale (July 23rd).