That games you found out that are pretty cool but they're not very popular...

Dark messiah of might and magic, great game imo and one of my favourites, multiplayer was cool but it's dead now. It had little population a few years ago...

The Last Remnant. I hate how the characters overreact sometimes and how silly the main character is, but I enjoyed this game and what I liked the most is that it has some cool sidequests, I think people should give it a go, really.

武林群侠传.
Wiki
Many well-known sites offer free downloads for the game.

A Game of Dwarves

Valdis Story. (http://store.steampowered.com/app/252030/)

It's beautiful and fast-paced, with a little of metroidvania all over. It's not perfect, it begs for a sequel and prequel. But yeah, I had so much fun playing it that I can't recommend it enough!

(PS: the platforming is hard. Especially the optional stuff. If you get stuck on some of these, just forget it and play the rest of the game.)

alpha protocol, duke nukem forever, aliens colonial marines. As far as I know none are popular, 2 out of three are pretty much reviled by the gaming community, but I love them all. Tons of fun to be had there no matter what anyonesays

Alpha Protocol

I would mention The Ship. It was bundled and shared so many times, that people simply think it's lame, or can't fing a working server despite on the forums there is a guide for it. In my opinion it's incredibly fun - you need to hunt your enemy, while being sure you won't get hunted down. It has a great, paranoid atmosphere, while you're hunting AND trying to look as harmless as you can be. :)

draw a stickman : EPIC is pretty cool too lol

Monster Rancher 4

It's nowhere near a perfect game but it brought so much potential to the series... F**k you Monster Rancher EVO!

Pixel Piracy Such Swag Very simulator

Rock of Ages!

not a music game, you actually play as a rock! x)

Etrian Odyssey series, it's on DS and 3DS.

Titan Quest too

Hard Reset is pretty cool.

Enslaved: Odyssey to the West is awesome. It's even 75% off ;-)

I really enjoy Anachronox, Advent Rising, NecroVisioN, Paranautical Activity, and Drunken Robot Pornography. They don't get nearly as much love as they deserve.

Mitsurugi Kamui Hikae , Vanguard Princess , Long Live The Queen , Go! Go! Nippon! , Agarest Generations of War , Agarest Zero , Infestation Survivor Stories , AfterFall Insanity

Guns of Icarus Online. Competitive multiplayer airship combat with a great community.

Anomaly: Warzone Earth.

A smartphone game that was ported fabulously to PC

Nail'd

Bulletstorm

Most indie games I've ever played.
Also, Super Robot Wars.
Sky Bubbles Deluxe needs more love in my opinion (Yeah, I know it's like Candy Crush, but with more stuff).
Contra: Hard Corps on Sega Megadrive/Genesis.
ROM CHECK FAIL, which is freeware.
Winigolf.

Always say Cannon Brawl when asked this question. In Early Access for about a year now, providing regular updates to a game that already stands like a house and that is a ton of fun to play is certainly no bad investment; great local-coop too which is a + for me personally

Cold Fear

Titan Quest gives the most content/euro of all games I've ever played.
Also are worth mentioning: Scratches, Undying.