How far back do you want to look, and how much tolerance do you have for antiquated graphics and interfaces? I can think of a a number of games that are largely forgotten and 6 or 7+ years old, but I know that some people don’t have the tolerance for what would now be clumsy by today’s standards.
If you dig survival horror, I cannot recommend System Shock 2 highly enough. It’s a first person RPG shooter. The engine is pretty clumsy, so it doesn’t play very well as a straight shooter like the ones you mentioned, but it is a fantastic story with great atmosphere.
They’re not that obscure, but since you listed adventure games, have you played the recent Sam & Max episodic games? They’ve now put out two seasons of about a half dozen episodes each. I’ve only played through about half of the first season, but they were pretty fun. Grim Fandango is widely regarded as one of the greatest adventure games, but is rarely heard of these days.
Also in the adventure genre is the Penny Arcade game, On the Rain Slick Precipice of Darkness, which is a sort of RPG/adventure hybrid. If you like Penny Arcade, it’s good, but if not, then skip it.
Good platformers are definitely a scarce commodity on the PC. How about Beyond Good and Evil? I hear nothing but good things about that one.
Mount and Blade is indeed a good game, if you like thoroughly open ended and unstructured gameplay. It’s pretty much the only game to get mounted combat right.
Dwarf Fortress is king of the indie scene, these days. I would describe it as a weird cross between Sim City, a roguelike, and an ant farm with ASCII graphics. Great for some, but very, very different from the examples you listed. Also, it’s free and constantly being updated.
Oh, Cave Story is an utterly fantastic 2D platformer in the vein of Castlevania/Metroid. If you haven’t played that, it is definitely worth a look see. One of the best ever free games, but I would recommend a gamepad.
Hmm, No One Lives Forever 2 is more of a straight up shooter, but it can definitely be played with an emphasis of stealth. An affectionate parody of 60’s spy films, I found the game to be quite funny, too.
The F.E.A.R: Extraction Point expansion is well reviewed, if you haven’t played that yet. The other one, not so good, it seems.
Any of those appeal?