There's a neat Reddit sub that is fairly new, if you want to have a look at people coding for "retro" consoles and computers, but in the modern day[0]. It's quite awesome.
Slightly off topic: it's not retro, but I have written and am currently playing with again writing homebrew for the GameCube, which is awesome fun :)
We used to have Core War tournaments in college.
It was first an assignment (write the VM) and at the end all the students warriors would compete vs each other and we watched the finals in the big amphi. One of the students even wrote an opengl visualization. Great memories!
Slightly off topic: it's not retro, but I have written and am currently playing with again writing homebrew for the GameCube, which is awesome fun :)
[0]: http://www.reddit.com/r/retrogamedev