Not technically a deal, but free Diablo 3 beta access for everyone - see post for details: https://forums.joeuser.com/422587

Grabbed this from slick deals:

Dungeon Siege III $5, Deus Ex $7 from amazon will activate on steam Starting tomorrow (Sunday) Amazon will have Dungeon Siege III ($4.99), Deus Ex Standard ($6.99), and Deus Ex Augemented ($7.99) downloads on sale. All three activate on steam. I believe this is as low as Dungeon Siege III has been and is 50% cheaper than the last steam sale ... granted I've heard bad things about DSIII but for the price may pick it up. Credit to Tony (amazon rep) for the info.

Yeah, Deus Ex: Human Revolution - Augmented Edition is a steal at that price.  Great game.

Dungeon Siege III is okay as far as RPG adventures go, but not the hottest potato going.  For $5 you can't go wrong.

Got a new one:

PC Digital Download Games: Psychonauts $2, Costume Quest $3, Stacking $7.50, Oh So Fine Bundle $10

Amazon has several PC Digital Download Games on sale. Thanks xanchui
Note, you will receive an online code to redeem via Steam client

• Psychonauts$2
• Costume Quest $3
• Stacking$7.50
• Oh So Fine Bundle (includes above 3 games) $10

Grabbed this from slick deals.  All games activate on steam. 

Amazon has some Paradox games on sale:

Majesty 2: The Fantasy Kingdom Sim for $2.49

Majesty 2 Kingmaker - Expansion for $4.99 

Mount & Blade: Warband for $9.99

There's quite a few more here.

Link

Mount and Blade: Warband is one of my favorite game of all time.  When I look at it on steam , it's funny to see the hundreds of hours I have played that game.  It is fantastic.  

This weekend, GOG has Alpha Centauri, Dungeon Keeper 1 and 2, SimCity 2000 and Populous 1 and 2 for $3 apiece.

- ripped from joystiq

Steam has Anno 2070 for 50% off.  That the most it has been discounted since its release, so I finally grabbed it.

http://store.steampowered.com/sale/ubi_weekend2012

There's a pretty good sale on other Ubisoft titles as well.

Sins of a Solar Empire: Trinity is 50% OFF on Steam! http://store.steampowered.com/app/201290/

Tropico 4 for $10.  Fantastic game. 

Tropico 4

OK - I've got a good one here.  First - it's free.  Second - apparently you can trade it in for a $5 credit for another game when you are done with it.  Third - its actually a really fun game.   Details through the link:  Disciples II gold for free.

http://slickdeals.net/permadeal/74608/green-man-gaming-disciples-ii-gold-edition-pc-game

Will you guys please drop the price on Twilight of the Arnor again soon?

Arnor is sold separately anymore, just the Gal Civ II bundle - https://www.galciv2.com/Purchase.aspx