Super Mario War

http://smw.72dpiarmy.com/

A single-screen fighting game a la Super Mario All-Stars

I'll copy the features here.

- Up to four players deathmatch fun
- a whole bunch of game modes (featuring GetTheChicken, Domination, CTF, ...)
- Comes with the leveleditor
- you can create your own maps...
- ... and a lot of people did so. There are currently over 1000 maps
- More fun than poking a monkey with a stick
- The whole source code of the game is available
- uses SDL and is fully portable to windows, linux, mac, ...
- CPU Players
- will make you happy and gives you a fuzzy feeling

Here's what it fails to mention. It's not online yet but it's their to priority supposedly. Also in their forums you'll be able to find the latest beta which allows you to play in actual worlds and comes with a world editor.