Most of us know what it is. Some dislike it, some like it. Pokesav is a program that edits a Pokemon Diamond, Pearl, or Platinum save file.
Its most famous feature is the ability to edit all aspects of a Pokemon, such as its ability, moveset, etc. Many people frown upon use of Pokesav'd Pokemon in trades and battles. Here, Pokesav is not supported as something to use in legitimate competitive battles and trades, but rather for fun and experimenting.

The Link to Project Pokemon's Pokesav Download Page is here: http://projectpokemon.org/editing/pokesav/pokesavdownload.php

Be sure to check it and Project Pokemon as well as this site!