Levelhead is a fantastic mario-maker esque platformer. The official campaign is a little over 10 hours long and is pretty good but its main draw is its incredible level editor and infinite number of quality levels online. I can't recommend it enough. Sadly it never got as popular as it should have but there's still a massive backlog of online levels to play.

Someone else mentioned Distance and I agree. It's a futuristic racing game with some horror elements. The campaign is short, but there's a great amount of levels in the workshop. The multiplayer modes are also pretty fun if you can grab a few friends (there's split-screen too).

Inkbound is launching from early access soon and while I wouldn't say it's the greatest roguelike out there, it's a lot of fun and very unique. It's essentially a co-op turn based RPG where you and other players play all your turns at the same time. I've played a lot of singleplayer too and the game feels well balanced there.

Voxelgram is Picross 3D for PC. Must-have for people who like nonograms.