Casual Collective with their robot battle games from 10 years ago. Though they did have more configuration, you could just add your name, pick a bot and play.

http://www.kongregate.com/games/CasualCollective/minions

http://everybodyedits.com was one of the first MMOs I played with no config (except a name and a map to join). In addition all of the maps were editable. So it was unique in that every map was completely user created.