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 leveling), 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 available 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 (don’t 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!")

For Warrior Maiden, bring a few MANA potions just in case, and if you wish, charcters that can cast Braveheart and/or Magic Boost. Firstly, use your characters to throw the 3 enemies across onto other panels[smaller "islands"] away from the main "island", usually your characters that can throw about 6 panels. That way, you can kill them separately without them damaging you, just don't stand too close to the edge. Most importantly, do not let Priere damage you, you'll probably get a one hit KO. It will take about 50++ turns if your characters are about level 450[My Lahari was lvl462 and the rest of the characters at lvl445+]. Priere is lvl 2000 and will join your team when you defeat her. One bad thing is that she only uses Monster weapons, but still, she is a good asset to the team until you are able to get Marjoly[level 2500].

The Mysterious Seal
Mana: 5000 Influence: 5000

This proposal unlocks Beauty Castle - and Marjoly.