I don't know if these are "toaster-level", but seeing as I can't even run the demo for SSIV, here's some of my favorites:
The Binding of Isaac (Twin-stick shooter roguelike)
FTL (Especially since the free expansion just came out)
SpaceChem (Best puzzle game ever)
Adventures of Shuggy (fun little platformer)
Terraria (Basically 2d Minecraft)
Altitude (2d arena flight shooter)
And Yet It Moves (Another interesting platformer with a great last world)
Braid (Puzzle game based around time manipulation. Really great as long as you ignore the clunky attempt at storytelling)
Cave Story (Classic action platformer)
Darwinia (Bizarre atmospheric strategy game)
Dustforce (Parkour ninja janitor platformer)
Frozen Synapse (Simultaneous turn-based strategy)
Toribash (Turn-based, physics-based fighting game. It's been greenlit on Steam, but it's available for free from the game's website)
Hotline Miami (Part stealth, part manic ultraviolence, feverish 80s-esque visuals and music)
Mark of the Ninja (The only stealth game I've ever enjoyed. Offers a TON of flexibility on how to approach the levels)
Race the Sun (Breakneck minimalist avoidance game)
Risk of Rain (Kind of like Binding of Isaac, but as a sidescrolling shooter)
Shatter (Physicsy Breakout-type game with awesome soundtrack)
VVVVVV (Challenging-but-not-too-frustrating platformer)