How do they pay for server cost? Someone told me slither.io had made it's owner millions. I'm assuming there are ads?

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.