While reading some of the questions about transcribing music here. I have come across these software tools that allow to slow down music and manipulate it freely. I think this is very interesting stuff and would like to dedicate this thread to make a kind of listing of the existing software with a description of what they can do.
It would probably be best to limit the answers to one program per answer and make the thread a community wiki. I think this can be a useful reference for when similar questions about transcriptions will come up again.