Not so well-known games, but really good

Adventures of Shuggy is pretty great too

We had a similar topic a year ago; http://www.steamgifts.com/discussion/v4F4R/underrated-games

I'll copy my own list;

Alpha Protocol

Freedom Fighters

Binary Domain.

I hate SEGA.

Oh, I remember the times I was playing Evil Islands all day long... It was such an amazing game! I really loved voice acting and actually the first sentence our main character says is still in my mind :D

As of my short list... (which may or may not be expanded later):

Art of Magic - very nice (and unique) RPG with strategy and action elements. You are a mage or summoner of some kind and you have to summon creatures that will help you in winning the game. It had good voice acting (from what I remember), many characters to choose from, interesting story mode and battle mode which was very entertaining as well. I also loved spells system where you had to choose between spells, it was a nice thing as your gameplay could be different every time you play the game.

The I of the Dragon - This is the only game on PC I know which gives player a possibility to become a true Dragon. Its action RPG style is awesome and if I remember correctly, the game is quite challenging and long. I really loved this one and I am looking forward to play it again, still have the old CD with the game :)

A Valley Without Wind - such a little gem that many players don't like and say it's boring or buggy... This game is definitely similar to Terraria and other such games as You build your base, craft things, fight etc. I would love to play coop with some people (or at least with one person) but so far everyone I met said that this game looks too bad or so...

I also wanted to add Arcanum but then I recalled that it's rather well-known game :P

Ringrunner: flight of sages
http://store.steampowered.com/app/258010/

One of the best games i have ever played

Stick it to the Man! - What a fun surprise this game was! If I'd known how delightful it was going to be, I'd have played it a lot sooner~!

Bad Bots
Before the Echo (formerly known as Sequence)
The Bridge
A City Sleeps
Contrast
Distance
Dreamkiller
Fairy Bloom Freesia
Fate of the World
Gray Matter
Hexcells & Hexcells Infinite & Hexcells Plus
Jets'n'Guns Gold
LYNE
Master Reboot
Race the Sun
Receiver
Sine Mora
Skullgirls
Spectraball
Symphony
Tengami
The Typing of the Dead: Overkill
Type:Rider
Velvet Assassin
Wrack

I really like Strike Vector, but is sad that it's not well know. http://store.steampowered.com/app/246700/

UnderRail
Great Fallout 1/2 style RPG. It's still in early access but will be fully released this year. Already content for dozens of hours. You can get it for a measly 8 Euro bucks on Steam. Highly recommended!

Original War. Awesome RTS game with fantastic storyline and music.
Outlaws (R.I.P LucasArts, it was the first game ever that "invented" sniper scope) Beautiful storyline.
Chasm: The Rift. A great experience dismembering bodies back in 90's.
Blood, one of the most climatic shooters ever made, but almost nobody knows it.
Interstate '76. Drive a Dodge Charger-like car with a gun on your roof, and shoot down enemies.

Here are some more:

• Binary Domain Criminally overlooked, cover-shooter with intriguing sci-fi story set in the future
• Cognition: An Erica Reed Thriller Interesting point-n-click, story-driven thriller with complex puzzles and plenty of plot-twists to keep things interesting to the very end
• Remember Me Well do you? Does anyone? Especially that the developer's Life is Strange is in the focus now?
• Obscure 1 Oldschool, classic Resident Evil style survival horror

Cognition was bundled before so should be dirt cheap, and I think Binary Domain landed in a bundle too(?)

Bedlam

Singularity, Alpha Protocol, Avencast, Stronghold Series.

Do check;
Famaze
Space Pirates And Zombies

ENSLAVED: Odyssey to the West - this game deserve a lot more attention than it got. I recommend this game especcially to action-adventure fans (i.e. Prince of Persia: Sands of Time). Great story, memorable characters and art style.