A bunch of stone cold classics in here already.
More for the list:
Daniel Remar's
Iji
Iji isn't much to look at, but it's been likened to 2D Deus Ex and I'm inclined to agree. It's a rare game that gives the player this much choice in how they build their character, and then genuinely makes every decision they made feel as valid and fully anticipated and supported by the game design as any other possibility. And then the narrative is responsive to your choices and actions too, on top of being pretty good anyway, and there's secrets and easter eggs and difficulty modes for days, so the whole thing is magnificently replayable. Music by HyperDuck Soundworks.
Everything else Remar has ever made is worth a look too, with Hero Core and Hyper Princess Pitch both being amazing in their own right.
Lyle in Cube Sector
A one of a kind metroidvania (which thankfully seems to still be available through archive.org) about saving your kitty with a powerset based entirely around grabbing, throwing, and conjuring cubes, backed by a funky soundtrack pulled from the tracker music scene.
Mega Man Arena
If you and a few friends are Mega Man fans, I can't think of any better way to kill a few hours.
