Recommend hidden Gems

Spec Ops: The Line

Just check my giveaway, last hour remaining!

Not sure if it's really "hidden", but anyway...

World of Goo! I f*cking love those little stretchy goo balls! It's a very goo game. I'v had such a goo time playing it. Goo times. Goo.

And another good game is Another World: http://store.steampowered.com/app/233550 It's 20 years old, but it didn't lost anything from it's value!

Critter Crunch! :3

Legend of Grimrock

Gemini Rue, I enjoy it very much and it's even in the current humblebundle :)

"Startopia" is perhaps the best Themepark-style management game I've ever played, perhaps my top all-time game, if I had to really think on it. Showing it's age yet again, but still beautiful and well-balanced. One of the few management games that I actually enjoyed playing the 'main missions' of. I actually bought a second copy of this on GoG just so I know I've always got one on stand-by for when my original CD crumples from over-use, haha.

"Sacrifice" is old and hard as balls, looks pretty gritty as it's visuals are showing their age, but still a solid RTS played from the perspective of a ground commander. Using the souls of the fallen units as the rebuild currency makes for a particularly wicked see-saw power-struggle. One of my all-time favourites, but one that can sometimes frustrate.

"Severance : Blade of Darkness" (or just 'Blade of Darkness' on GoG), is another older one, that despite it's age still manages moments of beauty with it's lighting system. This is the Demon's Souls from before Demon's Souls. You may need to take a few moments to rebind and adjust the key layout though, I remember it being a little awkward at first due to camera sensitivity and default layout. Definitely worth the hassle though. I could never decide whether I liked the Amazon or the Barbarian more. Dem weapons.

"Binary Domain" is surprisingly good, if a touch on the "generic shooter" side. I expected it to be a mindless romp but it was surprisingly satisfying to really have to rail the opponents to take them down. I normally hate "wiffle" shooters, where you can sink entire clips into enemies that just plain don't react until going through a death animation, but these robots actually work, as their armour strips down. The story actually had a little development in it, and by pure happenstance, every team selection I picked seemed to feel like it was "meant to be", like I was always the central focus of the story, and not missing vital conversation bits due to having selected the 'wrong' teammates.

Also, just because I can : "Spore". Yeah, the game itself isn't so hot, but I think it's delicious just because of how easy the creation tools are to use. An awesome outlet if you're like me, too easily frustrated and attention deficit to learn an actual 3D modelling program. It's hardly a hidden gem but there are still people who never heard of this who could sorely use an outlet for their ideas! Don't forget to hold tab while tweaking rig items, to get extra rotation options! Also use the mousewheel on the central parts of limbs to adjust muscle mass!

"Savant - Ascent" isn't much of a game, but you get about 5 included quirky mp3 tracks from Savant, in a genre that I think was described as "complex pop-dubstep". Catchy and odd, worth it for the music. During the christmas sale it's less than £1!

"Hammerfight" ate a lot of my time way-back-when. Steampunk flying machines that look like brass kettles with a propellor in the top and a free-rotation nut on it's side. Attach a giant medieval weapon to that hardpoint on the side, and then fly around using it's natural momentum to strike your enemies! Nothing beats the satisfying crunch of utterly oblitorating an enemy steampunk-knight by punting it with a giant hammer into a cliff-face, then watching it's battered and disabled husk tumble into a pit of bitey shadow-dwelling beasts. Seriously, at least go watch a preview or trailer or something. If you find the handling to be sluggish, manually alter the sensitivity settings (if it's still slow, move it to the other extreme of the meter, I remember this tricking me too).

3D Dot Game Heroes

beatbuddy for sure

Element 41 perhaps.

Some (No-so) Hidden Gems on Steam would probably be those found in the Humble Bundle store...

• Antichamber -- 1st Person puzzle shooter... It's an odd game to wrap your head around (worth checking out).
• Divekick -- One of the simplest, yet craziest fighting games I've played... What started off as a parody for a genre became a hit of itself.
• Limbo -- A moody puzzle platformer... so much storyline in without a single word of dialogue.
• Papers, Please -- One harsh game that's bound to kill anybody's cheery mood... but interesting to play.
• Retro City Rampage -- Loving Spoof of the 8-bit & 16-bit gaming areas in a GTA-style game.
• Cave Story + -- Great adventure game with the old-school difficulty spirit
• UnEpic -- A somewhat traditional RPG game with a LOT of nerd humor & references in it.

There's bound to be more, but this should get you started.

Waveform!

The Majesty series. Hardly read anything about it, but enjoyed the game for 600+ hours.

Be brute, beat the Wolfenstein 3D