These aren't necessarily quick games to play, or indie games. But they are largely unknown games, but each superbly excellent in its own way.
1 - Magi. I half half a strategy guide written for this somewhere. Two spellcasters square off in a hard-boiled duel. Amazing leveling up system - each victory costs you one year of your life, and each loss costs you 5. Eventually you die, and face off against death. The ultimate goal is to defeat Death, which could theoretically be done even with very few wins over the course of your life, if you level carefully. It's a bit austere graphically, which I find just emphasizes the fantastic spell balance.
Best of all, the creator is very active on the forums, and is already working on a sequel. Yes!
2 - Astral Tournament. Although Spectromancer is basically the sequel, and is arguably better, this first version was the one I discovered and played obsessively for years. It's an amazingly elegant spellcasting system - at once totally understandable and familiar, and yet unique in the game space. (Spectromancer made it just a tiny bit more complex, by allowing players to select targets for their spells. I think this tarnished the elegance some, though it does feel good to make choices.) A great tournament system allows you to lose as often as you win, and still keep chugging along towards victory.
3 - Space War Commander. This is a really hard core strategy game, put into a deceptively simple package. Earn resources through 3 different methods, buy ships, and defeat your enemies before your starbase degrades to nothingness. The really fantastic part of the game is how brilliant the level designs are - each one exposes a new facet of the game, the AI, and the useful attributes of each ship. If only I could play it in a window...
4 - Crimsonland. I never really liked Robotron in the arcades, and Geometry Wars just wasn't something I wanted to put enough time into. But this shooter pulled it all together for me, with weapons, upgrades, and a great theme. I hear this one is free now, to boot! I payed $20 or so a few years back, and don't regret it at all.
5 - Innerspace. A great top-down space shooter, this one uses your computer's filesystem as the solar map, and pits you in a battle against an imaginary virus which is eating your files. Best of all, you can ally or battle against ~8 factions which vie for dominance on your hard -drive. Will you ally with Internet Explorer and it's huge cache of files? Or will you stick with your games directory, and ally with all those .exe files? Don't forget to battle your inner demons to gain powerups. A classic.
6 - Kingdom Elemental. An amazing strategy game. I enjoyed buying troops and abilities a great deal, and battled my way through to the end - and then I tried to do it on hard. If I tell you it took me more than 60 attempts to beat the first level on Hard, you might understand how difficult this game became. I'm pleased to say that I eventually rose to the challenge, and defeated the entire game again. Unfortunately, a later patch drove the difficulty of the hard campaign way down, so that tremendous sense of accomplishment I felt might be lost to future generations. Sigh.
7 - Moonbase Commander. I wrote about this not long ago, but suffice it to say that this game beat out Scorched Earth for inclusion on this list. A great multiplayer game, with equal parts twitch and strategy.