The Venus Project
A server where you can do what you want, where you want. Just make sure to read the signs posted in spawn before you head off into the world.
Oh and realize, that everything you do on the server will have a consequence.
Example: Steal from another player, player might kill you. Wait, whats that? that player belongs to a city, that city marks you as a criminal and now, you can be killed and forced into punishment, or simply killed repeatedly by the city members.
second example: befriend some people, form your own city, trade with another city, grow and create a minecraft empire.
tl;dr : Its Survival, with no "rules".
tip: Don't drown when you spawn in.