BPM: Bullets Per Minute is a Doom-inspired roguelike FPS rhythm game. While that might sound like a random mishmash of genres, I hope developers like Awe Interactive never stop experimenting because BPM is a wonderfully unique experience. Right now, it's included in the Speedrunning with GDQ Humble Bundle, so you can get it for what works out at just $1.71 / £1.35.
RELATED LINKS:
Eleven great PC games not to miss in September
Doom becomes a rhythm game in BPM, coming to Steam this year