Your most recently played title that ya consider an 'underrated gem'

Well if it has to be the most recently played I would say Our Darker Purpose. One of those games that constantly tries to kill you but you will feel some kind of skill progression along with more possibilities to influence your game. It has a nice gimmick where completing achievements in-game will give you some kind of gimmick.
If not recently played I would add Fly'n and Full Bore. They all have good reviews but pretty view of them, especially Full Bore. It needs more love, it deserves it. For the latter two I've wrote a review if you are interested in them.

Zeno Clash 2. Deserves to be a lot more popular in my opinion.

Please dont hate me for this but im going to say bad rats. Now it might now be a "gem" but it is "underrated". Sure its bad but is it really the worst game ever? This question depends on the person asked.

AdVenture Capitalist! Kidding, kidding Seriously though, it's not recently played but I think that Knights of Honor is a wonderful game that oft gets lost between the massive colossi of the Total War series(especially Medieval 2) and Crusader Kings/Europa Universalis. It's a great "entry level" strategy game (although I enjoyed it even after putting probably 300+ hrs into my physical copies of EU 3 and M2 each) that still delivers a complete experience.

Hero of Many or Out There Somewhere.

Journey to Silius

Saint's Row 2 - 77%?! Really? It worth about 85-90%+! Good story, nice gameplay... Well... Much better than San Andreas!

Hero Siege. It's a rouge lite twin stick shooter. Very well done with plenty of characters to choose from.

http://store.steampowered.com/app/342620/ It came from space, and ate our brains

Deponia! Too bad that they messed with the translation in spanish (they changed the voices in the second part, and they didnt translate the last part T.T)

singularity. it's not underrated, more like obscure as it didn't get a lot or marketing when released, a pity because it's a really good game with nice mechanics and story. <3

also ibomber defense, a little tower defense game that i liked for some strange reason.

Mine is definitely Oil Rush.

Terrible name.

Terrible cover art.

But an incredibly fun game that deserves more attention instead of being a forgotten bundle game.

1. Stick it to The Man!. One of the best games I played in 2014. A must for people who like anything from Tim Schafer. I actually thought at first that the game was made by Double Fine, because the style of both the gameplay and dialogues was very similar to that of theirs. On top of that, the achievements are really fun to get and complete, and cards are included.

2. A lot of people didn't like Proteus, but I thought the atmosphere was unbelievable. It was also very thought provoking and made me smarter :P

3. For people who like co-op games, Outland is a very underrated game due to its poor marketing (came late to PC). However, it is a very solid 2d metroidvania game that you can play alone or with a friend (online coop is very rare with this kind of games). Includes cards.

4. Not exactly underrated, but Botanicula certainly needs more and more players to live in its surreal artstyle and soundtrack. My wife loves it, which is always a big plus to me :D

5. I see that you like noir games with interesting artstyle (and probably a similar gameplay to White Knight). You should, then, try The Detail. On the negative side, episodes are still being released for it. On the plus side, it has cards.

Well, if we're going with not just PC games.....

Black Sigil on the DS. The gameplay very much reminds me of Chrono Trigger even down to dual and triple skills, it has a decent story and is a lot of fun to play. Downside (and why a lot of people hate it) it has a couple of bugs, like random (but very infrequent) game freezes and being unable to get one of the dual skills.

Infinite Space, also on the DS. It's....very, very niche. It plays almost like a visual novel with space and ground combat. Tons of replay value though, you need to play through the game at the very least 3 times to see everything. Biggest downside is it's almost impossible to find.

Vampire the Masquerade Bloodlines

Max Payne 3 - game is interesting, but I even forgot why I deleted her after few hours.

Spec Ops: The Line
http://store.steampowered.com/app/50300

OK, here's one that I completely ignored because I didn't hear about it and it didn't catch my eye.
Gotham City Imposters really is worth a second look. I was turned on to it by TotalBiscuit, and it is now on my list of "things to play."

i've been playing Convoy recently...scratched that Mad Max itch till the real one comes out in Sept.

AdVenture Capitalist, lol that's the only one I can come up with.

Endless legend is my choice.
So few matches online :(

Dead Core, kinda went under my radar. It's a fun and challenging platformer, don't really care for the speedrunning aspects, but then you don't really have to. I wrote a review here: https://noisybark.squarespace.com/orix/2015/5/4/deadcore-review

Contrast
Indeed, the game is very short, but the game is very nice :)

Earth Defense Force 2017 & 2025. Just pure insanity, they're the only games I play that I don't mind when the frame rate drops low to like 10 fps due to the crazy amount of bugs, robots, blood, and explosions on screen.

Unepic(68) and Deathtrap(77) for +1