I've used goldwave for this. the results are always hit or miss, though.

It wont be perfect. The program chops up the waves. then stretches and nudges the pieces to make them a new freq. So , what you get is sometimes scrambled eggs.

If you are on a Mac, go to versiontracker.com and download the shareware called "The Amazing Slower Downer" I swear it is real.

Check out Wave Mechanics or Sound Toys. Here's the site: http://www.wavemechanics.com