Unlockable Stages

Disgaea: Hour of Darkness
Alternate Netherworld

Unlocked by passing appropiate bill in the Dark Assembly. Legendary Overlord Priere can be fought in the deepest parts of this dungeon and will subsequently join the party

Prinny Land

Used to unlock the different teirs of Prinnies. Unlocked by passing the appropriate bill in the Dark Assembly.

Cave of Ordeal

Unlocked by passing the bill in the Dark Assembly. Once cleared it can be revisted (in the PSP and DS versions) to play through the Demonhall Mirror, where Adell and Rozalin can eventually be fought as super-bosses. They do not join the party.

Human World

Unlocked by passing the corresponding bill. Unlocks the Human World Conqueror ending if cleared.

Beauty Castle

Unlocked with the Mysterious Seal bill. Marjoly the Most Malevolent Witch in the Universe can be fought here. She joins the party afterwards.

Baal Castle

The ultimate super boss Baal can be fought here. It is unlocked after beating the other unlockable stages. After beating him once, he can be fought in Prinny form, which is much more difficult to defeat.

Disgaea 2: Cursed Memories

Dark Sun World: Trigger 5 hidden switches within Holt Village with the X button. A new NPC named Friday MK II will appear. Defeat her party and she will now become a dimensional gate for the Dark Sun World. Dark Sun World maps are unlocked by performing certain achievements in storyline maps.

Land of Carnage: Defeat 16 different pirate groups for treasure maps withing item worlds (some of them only found in item worlds of legendary items). You may also steal those maps from them. After obtaining all maps, a brief cutscene will happen in Holt Village. Talk to Hawkeye to unlock the new ship.

Cave of Ordeals: Pass a bill in the Dark Assembly.

Makai Kingdom Clear 10 Dark world Maps.

Baal Castle: Pass a bill in the Dark Assembly after clearing 25 different Dark Sun World maps.

Disgaea 3: Absence of Justice

Land of Carnage: Pass Tiger Cave club bill with at least one of your characters in the club. This bill becomes available when you gain your first taboo ticket (from mystery room in the Item World or by defeating pirates in Item World). A new NPC will appear in the town. Clear 25 different stages.

Class World

Find the prinny that has an Afro. After clearing 3-3, you will receive messages after stages (repeated stages are OK for this) from that Afro Prinny has appeared. He appears in random spots.

You can then go into a character's world. The level of enemies will match the level of your character. You can only go into 10 stages at a time. The bonus list of the Class World only has random XP bonuses, so it is a good idea to finish each stage whenever possible. These maps are rather small and have a lot of enemies. The reward of finishing Class World stages is mana, which is based on the level of the character you entered Class World with.

Reverse Pirating

Defeat the pirates of an item in that item world, and have at least one member in the Reverse Pirating club. As long you are in Floor 20 or deeper in that item world, and can only do it once per 20 levels. You can capture or remove innocents for that item.

Hall of Ordeals

Clear Episode 7-4 and a bill will become available in the Assembly. Pass that bill to unlock 5 new maps, one of which is a nice power leveling map.

Extra Stage 1

Clear the entire storyline, then pass a bill in the Assembly for 1000 Mana. Clearing the map will cause Mr. Big Star to join the party. Salvatore will leave the party.

Salvatore Fight

After clearing Extra Stage 1, pass a bill in the Assembly for 2000 mana (there are 2 new bills that unlock after Extra Stage 1 that are both 2000 mana, one involves Salvatore, other one is Extra Stage 2). This fight involves Salvatore and defeating him will cause her to join the party for good.
Extra Stage 2

You must have cleared Extra Stage 1 to access this bill in the Assembly for 2000 Mana. You will go against a random human warrior type NPC. Clearing this battle will give you a Muscle World.
Extra Stage 3

You must have cleared Extra Stage 2 to access this bill in the assembly for 4000 mana. After passing the bill, you unlock a map that has Prism Red. Winning the battle will result in Prism Red joining your party.