Well it's not free but it's reasonably priced...
Brown Paper Tickets provides a ton of services for a VERY reasonable price. We used them for a couple of shows before someone decided we needed to waste a lot of money on an expensive in house ticketing system. I would be much happier just staying with Brown Paper Tickets. Their fee is 99 cents plus 2.5% of the ticket price. So a $10 ticket runs $11.24. You hide the fees in your cost structure and your customer never knows they are paying a fee. They handle assigned seating and your customers can even order online 24 hrs a day. They talk a lot about about being a "fair trade ticketing company"... not exactly sure what that means but I'll tell you they are a lot nicer to deal with than Ticketmonster.
Check them out it's a great way for a high school, college, or community theater to easily establish an online ticket system.