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

IWBTG(I Wanna Be the Guy).Hard and awesomeXD

Cargo!, funny game about building your own vehicles from various parts and kicking naked midgets in their asses so they will explode from happines (yup, it's true).

I think a lot of the puzzle platformers are pretty cool but not very popular. The one I have to mention here is Fly'n, just a beautiful game. And more recently another quite enjoyable game would be Full Bore. Both have pretty charming game characters.

Natural Selection 2.

Twin sector, cheap and very good

Poof

The Trine games. Theyre gorgeous and the game play isnt bad at all.

Magical Drop V - Too bad that last patch supposed to release will probably never come out.

Blackwell Deception. Always felt it was not getting the popularity it truly deserves.

• Sang-Froid - Tales of Werewolves - This game is awesome and you should already know that by now.
• Tiny and Big: Grandpa's Leftovers - The name is terrible, the game is really good and very funny too.
• Blockland - Many would consider it to be a Minecraft clone and maybe it kind of is, but who cares? The game is good.
• The Cat Lady - It's ok, my only "complain" is that personally I felt it could've been even better, so much wasted potential.

Anything done by Spiderweb

Castle of Illusion
DuckTales: Remastered

One Finger Death Punch

Q.U.B.E.

Unepic

Resident Evil Operation Raccoon City.

Land of the Dead: Road to Fiddler's Green.

Probably a bunch of others but right now their names are not popping into my head lol. I would say Vampire the Masquerade Bloodlines due to when I picked it up it was not popular at all and did not gain popularity much,much later on long past the death of the company that made it. Now its more popular so.. not sure if I can still put it here.. shrug

Battlefield Bad Company 2 (well maybe it used to be popular, but not now,
Napoleon Total War (the best strategy for me),
Command Conquer 3 Tiberium Wars (also very cool but I don't see a lot of people online),
Xenus 2 (should be very underrated, this is like Ukrainian Far cry, actually quite a cool game, give it a try if you liked Far cry)

velvet assassin

This is the part where I shill for Trackmania again.

For the ones who love addictive puzzle games, Obulis is really great. Most levels need logic, some also need skill to get a good timing but that part is not too hard in my opinion (a few real hard levels in the whole game).
On sale for a ridiculous price until end of summer sale : http://store.steampowered.com/app/11330/

All my early access games.

Brawlhalla

Minimum

Meltdown

AirBuccaneers

It's hard to say since it's all relative, indie games are less popular than AAA games, yet there are indie games that are more popular than others, so I could easily have a very long list but for now I'll just say...

Aztaka, awesome and unique game and one of the few where very few of my friends on steam have played it, it has some flaws sure, but the experience was one of a kind and I thoroughly enjoyed it

Drakensang: It's a great CRPG which borrows elements from Baldur's Gate, while using the (in my opinion) more interesting rule system found in The Dark Eye. It had some flaws, sure, but in the end, I thought it was more enjoyable than Dragon Age (this is not me complaining about Dragon Age by the way, because DA is also a stellar game. Drakensang suffered from non-existing marketing though, so most people know about it from word of mouth.

Unity of Command: An accessible hex-based operational level strategy game that manages to be surprisingly deep. I was glued to my screen during my playthrough of this game. One of the best of its genre.

King of Dragon Pass: This is beyond a shadow of a doubt one of the most unique games that I've ever played. It's like a mix between a CRPG, an adventure game & a strategy game, with non-linear gameplay, beautiful artwork and an interesting setting.