"Over 13 years ago, Doug Binks (mainly) and I started working on a shareware-style game called Avoyd.
A first-person shooter set in zero gravity, gamers could play deathmatches against bots or other people on a LAN. To our knowledge, it was the first game in which the players could build and destroy the entire environment." - Author's description
Note: This is an old version of Avoyd. Enki Software is currently working on a new version.
Download here (Windows)