I am currently a lease holder along with 2 other people. Our lease ends September 30,2010. I had paid for my last month (sept) $330, so did the other 2 lease holders. As we were not renewing the lease, my boyfriend and I decided to get a place of our own and I moved out Aug 7,2010, but had paid for all of August and all of September, as I am legally responsible. I stopped by the house to get my mail, and I found out that there was someone who had moved into the unit, and IS currently living there. The added person is not on the lease, and it was NOT discussed with the other 2 lease holders that he was moving in (the third lease hold had him move him regardless) so I asked for my last months rent back as I am technically replaced for the month of September.. but she won't agree to it. Am I in the right? And am I legally in the right to ask for 1/3 of what the new tenant paid?

Certainly - she can't "double dip." Take your ex-roommate(s) to Small Claims Court and PROVE she collected rent from you AND is collecting from the other person.