Luminous Corridor 0 [official site] is a top-down arcade shooter in which you run your 8-bit hero around a square screen, blasting away enemies and avoiding colourful explosions as you try to collect shining crystals. It's pay what you want and, from the short amount I've played, it's a lot of fun.
Once you get past the first few minutes it gets pretty hectic. If you accidentally shoot the crystals (which I did a lot in a panic) they explode after a short period of time and wipe you out if you're standing too close. So, you're trying to tag as many enemies as possible while making sure the crystals stays intact. Squares of the level will glow red and then explode every so often, while hungry enemies with massive mouths will streak across the screen and destroy anything in your path. The luminous environments change, too, blocking off certain areas of the screen and forcing you to adapt.
It's actually the third Luminous Corridor game and the first to feature local co-op. You can both play on the same keyboard, but it's recommended that one of you use a gamepad (although they're not natively supported, so use Joytokey to get it to work).
I had a bit of an issue with the controls: it's supposed to be WASD to move around and the arrow keys to fire, but for whatever reason mine had swapped over. It felt very odd to use my right hand to move my character, so I had to cross my hands over and play like a showboating pianist. Aside from that, it's good fun. It's created by Loren Schmidt and the sound is designed by Katie Rose Pipkin.
Get it from Itch.io for Windows, Mac, and Linux.