Pokesav is an editor program that allows you to hack your Pokemon Diamond and Pearl saves, adding rare Pokemon, changing their abilities, and doing all kinds of crazy hacky stuff. It appears that not only can you generate any Pokemon, you can then assign them abilities outside their type, and with more PP than the abilities can usually have. You need some kind of homebrew ability to do this, obviously, since you need to get your savegames onto the computer.
There's even a database of downloadable Pokemon, so you can get started cheating right away. We wouldn't recommend you use ill-gotten Pokemon online, but for your own use, we don't see why you should be bound by the limitations of the game.
Check after the break for a tutorial video sent to us by the Hall of Bards. Pokesav is serious business.