The answer to the OP is Reaper, the effect is - JS Stillwell Ozzifier. You can record
slowly in Reaper and then speed it up a bit to get those mega-fast raps and the Ozzifier
will double, treble or whatever you need (up to six voices) without all that mucking about
with 20 takes!
And ReaTune will do all your harmonies!
customers can sound like Busta Rhymes!