Recommend hidden Gems

Noitu Love 2: Devolution - $1.99

Giana Sisters: Twisted Dreams and it's expansion. :)
Or both... ^_^

This keeps going on sale, but Breath of Death VII x Cthulhu Saves the World when together (or separate) are solid goofy JRPGs.

The Wonderful End of the World is essentially Katamari Damacy with you as the ball. Fun times will be had.

Sequence is a triple-track DDR RPG battle. Enjoyment all depends on how much you like rhythm games or RPGs.

Demolition Inc. is a good destruction game, if you're like me and tend to wreck anything/everything in games with breakable buildings.

Anything else I could name would be debatable as being 'hidden' considering how popular random titles are.

I'm looking at these ones currently.

Super Laser Racer, Swarm Arena, Solar Flux

The Secret of Monkey Island: Special Edition
http://store.steampowered.com/app/32360/

Alice: Madness Returns, it's also supercheap in Europe, only 12.50€ = 17$ ^^

Sang-Froid - Tales of Werewolves

Excellent Strategy and Action combined.

Sang-Froid - Tales of Werewolves

Is Altitude lesser known?
Because it is like the best multiplayer game of all time.

Nation Red.

Recettear: An Item Shop's Tale (Store sim+Dungeon crawling+Great writing)
Mount & Blade: Warband (It's own genre)

Fist Puncher
Foul Play
Knights of Pen and Paper +1
Teleglitch Prepare to Die Edition
Bleed (Did an interview with the creator of this indie game its so much fun)
Symphony
Syder Arcade

Ben There Dan That and Time Gentlemen Please. I throw a copy of the pack in my cart to give away whenever I buy something during the sale because I like it so much.

Recettear

Unholy Heights

Cook, Serve, Delicious!

Not sure how unknown it is but, Crusader Kings 2

among others.

I love this thread. Thanks everyone who posted. :)

Expeditions: Conquistador

One of the best indie games of 2013 and one of the best turn based tactics/RPG hybrids I've played in years.

Evil Genious

Tiny Barbarian DX.

I hope I'm not the first to say this, but the S.T.A.L.K.E.R. series is amazing.

Original war

I'll throw out Jade Empire. Lesser-known BioWare RPG that falls somewhere between KotOR and Mass Effect plus a healthy dose of button mashing beat 'em up in terms of gameplay in a non-specific far east/fantasy setting. Not quite as massive as some of the others, which may be a pro or con depending on how much time you have to sink into a game. Probably not my favorite of their games, but it's near the top of the list. Also often gets left out when everything else goes on sale, but is currently 50% off (and may yet get a daily/flash sale).

galactic bowling (also zombie bowl o rama and elf bowling) pretty fun action takes on bowling with powerups
caster but its not on sale currently :/
hinterland also not on sale :/ severely underrated
Weird Worlds: Return to Infinite Spacereally quick and fun rpg space game
Droplitzreally entertaining casual game
The DIGamazing classic adventure game, I dunno if its unknown anymore but I never see it mentioned
Hegemony: Philip of Macedon surprisingly good total war look alike game covering one of my favorite parts of history thats often overlooked.
Railroad Tycoon II not sure if this is a hidden gem but it is an amazing tycoon game, up there with the original rollercoaster tycoons.

Project Zomboid.

Septerra Core: Legacy of the Creator

The dream machine, desperados, to the moon