The first that comes to mind when thinking quality freeware is
Armagetron, the most fun I've had with a Tron lightcycle clone. It might be an insult to call it "low rent", as a ton of effort has clearly gone into it, but the game's definitely of a low profile in the grand scheme of things. Seems to have an active and loyal community and deservedly so.
BreakThru! is a simple, unglamorous puzzle game available on a number of different platforms that's surprisingly entertaining and super relaxing. It's not much to look at, I passed it over for years expecting it to be thoroughly boring but took a chance on a $1 Game Boy copy I stumbled upon last year and it's rarely left my GBA since.
Future Tactics: The Uprising, despite its bargain bin exterior, is a really cool turn based stategy game and multiplatform release from a generation ago that seems to have (somehow expectedly) been largely forgotten. I remember the gaming media of the time shrugging the game off with 6s and generally being apathetic but I seriously dug/dig it.
Break 'Em All on DS may be just another Breakout clone destined for the clearance rack but I've had a lot of fun with it. Overall, I'd say I prefer it to Arkanoid DS.