Application Annihilator does the same thing as some commercial products for free. Just select the application and Application Annihilator will automatically remove all the the files on your hard drive related to that application: the preferences, support files and of course the application itself. And if you change your mind, just go into Finder and Undo the last operation, and everything is restored to its former glory.
Mac OS X 10.4 or later
*Previously available here