Disgaea: Hour of Darkness/Bonus Worlds

A number of extra regions can be opened via proposal in the Dark Assembly. In order to do so you must have unlocked the proposal, have the proper amount of Mana available, and have enough influence with the Assembly (or be able to grind them beneath your boot-heel).

Prinny Land
Mana: 400

Area Map: Prinny Land 1, Prinny Land 2, Prinny Land 3

The Prinny God has a Testament that you can steal every time you return to this stage, making this a good place to get powerful equipment for your team.

Cave of Ordeal
Mana: 700

Area Map: Cave of Ordeal 1, Cave of Ordeal 2, Cave of Ordeal 3 (one of the best spots for lvling), Cave of Ordeal 4, Cave of Ordeal 5 (make sure that you can reach the zombie king by having 2 characters that can throw 6 squares away or you'll never be able to finish this map), Demon Hall Mirror (I think it's only availible in the ds version and the psp version)

Demon Hall Mirror: By the time you fight Adell the second time and Rozalin, you will thoroughly hate this stage. All the enemies have insane amounts of life and always have some geo symbols to turn the tides even more. The hardest ones are probably the one with the four clone symbols (each one creates a clone) and Adell and Rozalin with 3x boost and 20% recovery (They have around 2 and 4 million HP, respectively).

Human World
Mana: 1200

Area Map: Central City, Neo Eden, EDF Headquarters(dont forget to steal the astro suit before killing General Carter)

Alternate Netherworld
Mana: 3000

Area Map: Alternate Netherworld, Hall of Sin, Hall of Penance, Malefic Seal, Warrior Maiden(Priere "Roasted Pork!")

The Mysterious Seal
Mana: 5000 Influence: 5000