Open source (as in BSD, MIT, WTFPL) are more liberating than "Free Software" (GPL). They greatly enlarge your opportunities in how you use the software.
BSD, MIT and WTFPL are all on the list of officially verified Free Software licenses: http://www.gnu.org/licenses/license-list.html and are thus "Free Software".
Open Source, similarly, is defined as software released under a license that follows the definition of Open Source Software ( http://www.opensource.org/docs/osd ). The OSI also has a list of approved licenses: http://www.opensource.org/licenses/alphabetical