Today I have added my old project SI to this site, since some oddballs find it a bit cool. It is a space invaders clone for MS-DOS in 287 bytes that I made years ago.
Originally, it was meant to be a submission to a 256 byte space invaders competition, but I never got it down to 256 bytes. All the entrants that made it had used text mode. Using a graphics mode (320x200) was a bit overkill since the graphics data and the drawing routines became a few bytes too large. Let me know if you can optimize it, even as little as one byte.