Deltascention

Deltascention is an action roguelike takes place in a single room, but that doesn't mean that there isn't hours worth of fun stuffed into it. 
The game varies over 50 waves with a final boss at the end. Every wave, new enemies spawn and the arena changes to a new layout previously created in the level editor. Killing enemies gives Xp that can be used to grant a choice of 3 upgrades of varying rarities, maybe even a new weapon. At wave 50, if you don't want to fight the final boss just yet, you can wait a while and loop for another 50 waves with a lot stronger enemies to get new upgrades.
A large selection of different weapons to choose from along with a ton of unique upgrades. Choosing a weapon even allows you to salvage it for a special one time upgrade! The salvaging upgrades are reset after changing weapons however. 
The arena is one single elevator shaft, flying upwards. The ground is made up of a 16x16 grid of 5 unit tall pillars, along with a 2nd layer of single unit tall cubes. The arena is controlled by .txt files that can be located in the game files. This allows players to copy the files, and send them to their friends over almost ANY messaging program. Then they can just drag them into the files, and the new arena is there. But, it does require both layer 1 and layer 2 files, otherwise it will not work.
Enemy waves get progressively more difficult over time with new encounters arriving every few waves. Some enemies should take much higher priority than others. Some are just fodder, and some could end a run if not dealt with quickly.