Any will do. Actually you can even use the sound editing/mixing softwares to do the same - the only problem with them is that you don't have features like start and end markers, looping etc.
I have used Amazing slow downer (by some Romi / Roni) which is a trialware :(
Audacity (freeware) has a slow down feature - but is pretty bare, as I said above.
Lemme check my home PC to see what is kept there ...