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.

Twin Ruin , twin-stick gem laser focused on simply intense arcade play, plus a dual color mechanic.
Radio Free Europa , a clever space shooter mixing thruster and free movement, with lots of guns.
Devastator , another twin-stick from the great radiangames dev of the Xbox indie arcade days.
ArdeinFall , 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 great and more than a little cracked gem of the genre that's way cheaper than it deserves.

Recent escapes from Steam Gulag:
Devader , a top-tier gem of an arcade twin-stick, full of clever play, weird enemies, and ideas.
Cavity Busters , a 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.