Lots of good and great niche games don't get acknowledged as real games by Steam's internal metrics, barring them from profile showcases, despite desperately needing attention and visibility. This space is dedicated to a few of those unfairly hidden gems.

Nova Swarm - Fantastic little gallery shmup with weird bosses and a ton or personality.
Twin Ruin - Twin-stick gem laser-focused on simply intense arcade play, plus a dual color mechanic.
Radio Free Europa - Clever space shooter mixing thruster and free movement, with lots of guns.
Astral Gunners - Solid horizontal scrolling shmup with cute patterns, several ships each with a different defensive ability, and a bunch of variant rules to alter the game. Cartoony looks but serious game.
Devastator - Another great twin-stick from the great radiangames dev of the Xbox indie arcade days.
Ardein Fall - Top-down style defense shooter with several interesting mechanics and procedural runs that stay fixed each day.
Gravity Ace - A rare lander game with graded levels and workshop for user-made levels.

Speaking of twin-stick shooters, AtomHex [www.incitti.com] comes from outside Steam, but it is another neat and more than a little cracked twin-stick. Might not be able to upgrade this from its shareware demo version anymore, but still playable.

Past escapees from Steam Gulag:
Sektori - The best arcade twin-stick shooter to come around in a long time. High intensity.
Devader - Top-tier gem of an arcade twin-stick, full of clever play, weird enemies, and ideas.
Cavity Busters - Crazy top-down shooter style roguelite with really rich core game-play.
---Red---Tether--> - Like a top-down shooter but you fight with elastic cables. Did I mention you fight with elastic cables ?

Also, I run the Steam curator 8cade: The Infinite Arcade if one wants more recommendations in its domain space.