This is a legendary tool developed by the author of The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws (2nd edition).
Unfortunately there is
no native Mac Os X version but Corsaire packed one. Since they updated the site, the link to download it provided in the aforementioned book is broken, so I will provide a new working one of the recently released version 1.5 1.6 of Burp Suite Free.
All rights reserved to Corsair and Portswigger.
See also here for a [much less powerful] alternative.