Pokemon XD: Gale of Darkness: Mew
When you transfer Mew from your GBA cartridge over to your Pokemon XD file, you can teach Mew moves that it could not otherwise learn. You can do this by having Mew in your party while talking to the man who wants to see "the Pokemon that can remember all attacks". After you speak with him, he will ask you some questions. If you answer all of them correctly, he will show a list of moves that he will teach your Mew. You can only choose four moves, though.
Answering questions.
Battling with Mew.
