Travesty! I've just realised that we've never linked to the utterly superb bunker-architecting side-scrolling four-player battle game Cortex Command. Luckily IndieGames.com feature page averted this catastrophe, and I can link you guys to the demo.
Cortex Command is a 2D base-building game in the vein of Worms, only with robots piloted by disembodied brains and giant bunked filled with electronic death. (Also the astoundingly detailed pixel-art and physics makes Worms look a bit like it was made during a sneezing accident.) It does require a bit of patience, however, so take your time and experiment. This one is highly recommended.