Time to quickly steal from myself and make some suggestions here. I actually have a bit of homebrew that I think would be worth porting that I posted in the general request thread, but I figured I'd post here.
The first bit is a fangame called
Super Mario War. While development on it has stopped and the last publically released version was 1.8 Beta 2, it's a lot of fun to play! It's basically deathmatch Super Mario where the goal is to defeat your opponents by stomping on them. In addition to that, it has a lot of other nice game modes. The game has been ported to a lot of devices that are homebrew-capable, including the Nintendo Wii and Sony PSP. The source code and download links to releases for Windows/Mac/Linux/Wii/Everything else is available here:
http://smwstuff.net/game
Super Methane Bros. would also be kind of cool. Despite it's name, it's not a Super Mario Brothers clone but rather a clone of Bubble Bobble. The idea is to gas your enemies, suck them up, and then spit them out against a hard surface. Your goal is to get rid of every enemy in a room in order to progress. It was originally a commercial Amiga game, but it was later open sourced and it can be found here:
http://methane.sourceforge.net/
Another suggestion that just popped to mind is
Wolfenstein 3D. The predecessor to Doom, it's still fairly fun to play and is a bit more "stealthy" than Doom I suppose you could say. It hasn't aged as well, but there's still some fun to be had with this title and I believe a shareware version, similar to most FPS games at the time, was released. The source code for the original game is present on Github:
https://github.com/id-Software/wolf3d