As usual, Steam is hosting a sale. However, let's start with a more useful piece of information: The Steam Greenlight Submission Fee is going for $50 (50% off).

Also as usual, I'm trading a lifetime Fooldom Come subscription for any game off of my Steam wishlist. Fooldom Come is a passport for benefits in my BYOND games which currently include Flesh And Drone, Fused Finale, Gears & Peers, Grim Prospects, GrimTunnel, MeanderGall, Occupied Forces, PathWrath 2 and Wrangle, Warp And Weep. I also failed to return Fooldom Come to its normal price after the last sale so I'll leave it at $3 (66% off) for now.

(For anyone interested in trading, I also have a copy of Frozen Synapse which is currently going for roughly $6.25.)

75%+ Off:
Madballs in Babo:Invasion $1.24 ($3.74 four pack)
Garshasp: Temple of the Dragon $1.24
Critter Crunch $1.74
Offspring Fling $1.99
Cargo! The Quest for Gravity $2.49
Cargo Commander $2.49
Gridrunner Revolution $2.49
Gatling Gears $2.49
Cave Story+ $2.49
Serious Sam 2 $2.49
Serious Sam 3: BFE $9.99 ($29.99 four pack)

Just plain cheap:
The Basement Collection $1.99 Gifted!
Adventures of Shuggy $2.49
1000 Amps $2.49
Titan Attacks! $2.49
Gateways $2.99
Dynamite Jack $2.99
...Well, the Greenlight fee was 50% off. Apparently, that was temporary.

(Also, SW:KOTOR II was a flash sale so I removed it from the list.)
I bought the portal one and two bundle, the witcher enhanced edition, and to the moon. I beat portal one a couple years ago but never played the second, so im looking forward to going through those. The witcher is something ive always been interested in, but never had. As far as to the moon, I hear its a really emotion invoking indy title.
Here are some 75+% games off that I already own and can vouch for. Serious Sam Double D and Nation Red might be the best bets for most. Caster might be great for any Dragon Ball Z fans left around. Altitude doesn't seem to have a very active community anymore, but there are bots and I grew used to Schnitzelnagler shooting me down.

Caster $0.99

Faerie Solitaire $1.24
Garshasp: The Monster Slayer $1.24
Who's That Flying?! $1.49

Nation Red $1.99
NightSky $1.99
Serious Sam Double D $1.99

All Zombies Must Die $2.49 (Craft your status items ASAP.)
Altitude $2.49
Mr. Robot $2.49 (No custom levels on Steam.)

Amazon is also selling a bundle of Bioshock and Bioshock 2 for $4.99. It comes with Steam keys.
Wow nation red looks pretty cool I might check that out its only 2 bucks. Not sure about serious sam I played the second one on the original xbox and it was entertaining, it looks odd in 2d though. And the bioshock deal sounds cool but I just beat those a couple months ago. I might run through them again though for when the new one comes out in 2013.