Run a search for "ticketing software" and see what comes up. There was a very informative thread a while ago.
Something we're looking into is this one:
Seat Yourself - Online Ticketing - Online Ticket Sales Software System. You don't have to pay for it, a $0.50 fee is added on to each ticket. I believe, though, that you do have to pay credit card fees if a customer pays via credit card.