GMOD and MINECRAFT server:

You could always create two gamemode worlds, with a portal between them.

It is some mod, I don't know what it is

That sounds pretty cool!

Yeah I saw it on a survival/build server.
Worked pretty well I thought.

Here, I think this is it. Check it out

http://dev.bukkit.org/server-mods/worldmodes/

Edit: Hmm...actually, I am not sure this is it. Looks like this one is more of a command plugin that changes a player's gamemode from survival to creative or visa versa.
In addition, it does not allow items to be passed over, like if someone is on creative and gives himself 64 diamond blocks, they will not switch over to the survival world.

I don't remember the server that had the two world server portal, but I remember it was an obsidian border with a lava portal entrance. Maybe it is an extension to the sign teleporter mod.

EDIT: HAHA! I have found it! Link here:

http://forums.bukkit.org/threads/edit-misc-tp-fun-multiverse-2-2-world-management-bukkitdev.3707/

Lol, check this plugin out.

http://forums.bukkit.org/threads/mech-rpg-dragontravel-beta-1-7-be-a-dragon-rider-1818.46079/

Oh, and this one seems cool too. Sorry for all the plugin suggestions, just they all are so cool!

http://forums.bukkit.org/threads/admn-econ-rewardme-reward-your-users-with-custom-rewards-1-0-1-r1.43751/

nice find.