Bukkit

Enforces mob limits per chunk on mob spawners.

Prevent mobs and dropped items from transporting via nether/end portal.

Repair items with the workbench and keep enchantments.

Allows signs to be edited with ease

ExchangeMarket lets players post buy and sell orders to be matched by other players.

Lifestones are devices that players can attune to. When they die they're resurrected at the last lifestone they used.

Transmits a redstone signal wirelessly

Charge players exp points when placing blocks at specific heights.

Changes enchant costs from levels to XP.

Notifies players when someone enchants an item.

Toggle the 'hardcore' setting in a world's level.dat file.

Create and restore snapshots of player dat files.

Notifiy admins when certain commands are used.

Teleport players stuck in a portal to spawn.

Keep chunks and their mobs loaded if a player died recently.

Cause mob breeding to drop spawn eggs.

Locate nearby ChestShops buying/selling an item.

Causes killed mobs to have diminishing returns on drops, increasing other mobs rates to compensate.

Allows admins to keep notes on players.

InvisibilityViewer allows players to see invisible (effected) entities.