Not all Worlds created in LEGO Universe were intended for public release. Developers and LUPs created many worlds throughout the course of development, ranging from personal test worlds to do whatever they wanted, to more extravagant gameplay and art tests. There were also "Game Challenges", which appear to have been internal contests for things such as world design.
Due to the massive amount of worlds like this, and that few have been shown publicly, this is not a complete list.
Systems Testmap (Map 1)
A simple test map, presumably updated many times throughout development to test whatever developers were working on.
Pets Test (Map 4)
A test map containing many Pets.
Pirate Ship Map (Map 13)
A small island with various pirate ship models. Oddly, the game's database lists it as a "Test Racing Property".
Ninjago Test Map (Map 30)
A world for testing various things related to Ninjago, though the game's database lists it as an in-development Nimbus Station map.
Racing Test Map (Map 314)
A test of a "new race type". The database for the final publicly released version of the game gives a display description of "Gate Rush" for this map, but it is possible that this racing map replaced whatever had been there previously, as the video recorded of it is from an in-development version that was never released.
Blocky Mtn Airport (Map 325)
An airport world created by a LUP team named "lupowski". The same team created things for a parachuting system in a Game Challenge, there are what seem to be skydiving targets in the airport. Two parachuting-related NPCs exist: Dirtnap Phil (a Parachute Instructor), and Dirtnap Barski (a Parachute Game Vendor).
Super Awesome (Map 377)
A test world very reminiscent of the Spider Cave. It is unknown if this is simply a map made for fun by a developer, or an exploration of ideas for the Spider Cave. Its main file is located at Test/super awesome/super_awesome.luz, implying some relation to map 400.
Artist Test Map (Map 387)
A test map for Ellory Gillis-McGinnis, an artist who worked on the game.
Dead Man's Bluff (Map 396)
A pirates-themed Game Challenge world. A pirate mission giver at the beginning gave players a mission to rescue a captain across a lagoon. At the end of the world, the captain revealed to players that they had passed his test, and offered to let them join his pirate crew. Enemies included "Darklings" which used the skeleton hybrid model and had pirate hats.
Design Test Map (Map 400)
This map's main file is found at Test/super awesome/superawesome.luz, implying some relation to map 377.
Steve's Attack/Defend Test Map (Map 420)
A capture-the-flag style PvP test map, similar to Forbidden Valley Siege.
Megan's Haus of Horrors (Map 512)
A test map for graphics programmer Megan Fox.
Pirate Side Scroller (Map 525)
A sidescrolling level primarily using Gnarled Forest assets, created by level designer Steve Young. Notable for having climbable walls, a mechanic not seen in any released worlds.
Old Travel Hub
An "old travel hub", containing signs that can teleport the player to various early worlds. Possibly map 315, "Transport Hub".
A Game Challenge minigame made by "Team 7". A Wizard NPC (whose textures are reused from Wizards in Nimbus Park) gave a mission to repair a pinball machine, which players could then play. Comically, a minifigure's head was used as the pinball.
Though the name is unknown, a volcano world was created by the A-Team (the team of LUPs who created MoonBase) early in development, according to one of its members, Jordan Schwarz. It had invincible lava orc enemies which would throw lava/fire at the player, and bore a resemblance to LEGO Agents set 8637 Volcano Base, despite the world being designed about six months before the set was released and having no official relation. Another world created by the A-Team's name was "A-Team" spelled backwards (Maeta or Maet-a), though the content of the world is unknown.
Outpost was a world being created by the LUP team Sector 6. The world was not completed in time to be included in the live game, but footage can be seen in a video focusing on team member Steve Barker, and concept art and more information can be found on the blog of another member, Zipblock:
2008 LUP Event Worlds
At the 2008 LUP Event, the attending LUPs were taught how to use Happy Flower to create their own worlds. Many photos of the event were taken by Team Deerbite member Alfred Spitz (some of which are shown in the gallery below) as well as some videos. It is likely some of the worlds shown were created by developers as examples, or possibly that some of them are even early versions of worlds like Gnarled Forest/Forbidden Valley.