C-Dogs SDL is a classic overhead run-and-gun game, supporting up to 4 players in co-op and deathmatch modes. Customize your player, choose from many weapons, and blast, slide and slash your way through over 100 user-created campaigns.
It is released under the GNU GPLv2, and has downloads for Windows, Mac and Linux available on itch.io. It is actively developed, with 30 contributors and 3776 commits, the last commit being 2 days ago.
Edit: For some reason the webm screenshots aren't showing up, so for those go here.