Daily deal on GamersGate is <span class="bold">Unity of Command Trilogy Bundle</span>, discounted by 85%. The game and its DLCs are using offline serials for activation, but you can also use those serials to activate the game on Steam.

Retroism is currently having a labour day sale until Sept 8.

All/Most games activate on Steam (I have not checked every one, but all the ones I did check are Steam):

Games under $1:
1942 Pacific Air War
Across the Rhine
B-17 Fighting Fortress
Bloodnet
Crazy Chicken Invasion
Darklands
Dragonsphere
Eradicator
F-19 Stealth Fighter
Gunship!
Nam
Redline
Shadow Ops Red Mercury
Slave Zero

Also, get a free Classic Explorer Pack (Sid Meier’s Pirates! and Pirates! Gold Plus, Sid Meier’s Colonization, Sid Meier’s Covert Action) with the purchase of one of the following:
Bladestorm Nightmare
Dead or Alive 5: Last Round
Dynasty Warriors 8: Empires
Dynasty Warriors 8: Xtreme Legends Complete Edition
Ninja Gaiden 2: Yaiba
TouKiden Kiwami

Also: Save $10, and get 3 Scenario DLCs free when you buy:
NOBUNAGA’S AMBITION: Sphere of Influence ($49.99 discounted)

Hyperdimension Neptunia Re;Birth1 and Hyperdimension Neptunia Re;Birth2 both 60% off ($11.99 each discounted)

Zotrix is 25% off ($9.79 discounted) and get a free Hyperspeed key (Steam).

$4.99 on Humble (75% off) https://www.humblebundle.com/store/p/payday2_storefront

I used this to find out: CheapShark

...and not that anyone will give a fuck, but UPLAY has some deals on.

Has anyone bought the Middle-Earth Shadow of Mordor from Bundlestars bundle? It almost seems too good to be true, 15 euro for the GOTY edition. Any idea about the origin of the key nation wise?

More offers on Humble Store's End of Summer Sale.

Tower of Guns (DRM-Free/Steam) flash sale -80% for 10 more hours.

Atari Sale, up to -75%. All the games require a Steam account, with the exception of Atari 80 Classic Games in One, which is DRM-Free.

Build Your Own Paradox Bundle. Select 3 games for an 82% discount, 4 games for an 86% discount, or 5 games for a 90% discount. All games require a Steam account.

Build a Daedalic Bundle. Select 3 games for an 80% discount, 4 games for an 83% discount, or 5 games for an 85% discount. The vast majority of games are both DRM-Free+Steam, except for the Cultures games and Blackguards Deluxe Edition, which are Steam-only and Gomo, which is only DRM-Free.

I've wanted Slave Zero since I saw a magazine ad for it, possibly even in 1998 before it came out.

That one is on sale on Steam right now too, CDN$1.09, a better deal than retroism at the current exchange rate. (http://store.steampowered.com/app/328470/)

If you haven't bought it yet, I have a left over steam key for slave zero from a recent indie gala bundle.
It's yours if you want it.

This would be nice to see on GOG as well since it doesn't get much older when it comes to really classic games, it even has "Pong" included in the package. ;)

Nice. Does anyone know if the extras of the retail version are included in Humble's version?

What extras are you talking about? Are they in-game or external?

According to the IGN review the retail version contains some bonus material like images of original box art and cartridges, game documentation scans including some DC comic books and a short series of video interviews with Nolan Bushnell.

http://www.ign.com/articles/2004/01/28/atari-80-classic-games-in-one-review

Let me install my backup and I'll get back at you.

Yup, it does include the bonus material.