From Bulbapedia, the community-driven Pokémon encyclopedia.
Screenshots of Pokésav 0.39 English, when used with Windows XP
Pokésav is a program that edits save files for Pokémon games, developed by a Japanese programmer by the name of COM. There are currently versions of the program for editing the save files of Pokémon Diamond, Pokémon Pearl, Pokémon Platinum, Pokémon HeartGold, SoulSilver, Pokémon Black, and Pokémon White games. Save files may be backed up from a commercial cartridge using a flashcart such as the R4 or a cheating device such as the GameShark Duo. Pokésav can also export Action Replay DS codes that, when activated, will provide the same effects in-game as editing the save file.
As of October 31, 2010, the latest Japanese version of Pokésav is 0.06c of the program for Black and White, and there have been no new versions since. This may be influenced by the increasing threat of Japan's internet piracy laws, as well as the increasing use of two similar programs, PokeGen and PKHeX.
With Pokésav, players can edit many aspects of the game including Pokémon and their movesets, the inventory, and in-game records. Use of the program does carry a risk of corrupting the save file, however. Backing up the save file before using the program will mitigate this risk.
When the early version of Pokémon HeartGold and SoulSilver's Pokésav was released, Sinnoh location names were still in the program, making it difficult to set some values. While this would later be fixed in the Japanese version, the current English version (.03a) has not made this update.
The latest version of Pokésav for Black and White (0.06c) does not edit Gym Badges.