SpawnerEdit makes editing your mob spawners easy, no commands required! Simply right-click the spawner to open an inventory window. You'll see a spawn egg in the enchanting slot; replace it with a different spawn egg, and voila! Your spawner will now spawn the mob associated with the new egg.

Currently you need to swap mobs by holding an egg and clicking the one already there; otherwise it won't let you remove the egg.

To edit a parameter of the mob spawner, click the egg while holding a stack of the appropriate material. A larger stack allows you to change the parameter in larger increments. There's no limit to how high you can set the parameters, so be careful.

The permission "spawneredit.edit" determines whether a player can edit spawners. By default everyone has permission. Currently there's no configuration required. The current value of each parameter is displayed on the enchanting buttons in the order spawn limit, max delay, min delay (from top to bottom).

Features summary:

Edit mob spawners by right-clicking (permission: spawneredit.edit)
Edit minimum delay between spawns by adding glowstone dust (permission: spawneredit.edit.mindelay)
Edit maximum delay between spawns by adding emeralds (permission: spawneredit.edit.maxdelay)
Edit spawn limit by adding gold nuggets (permission: spawneredit.edit.limit)
Permission spawneredit.edit.all covers all three of the above

Possible future features:

Edit the delay between spawns
Spawners drop eggs when mined (though you can do this already with any drop-editing plugin, such as DropFactory)
Temporarily disable spawners entirely by removing the egg
Configurable parameter-editing materials
Edit spawn count