Easy On-Line Event Ticketing?

Ronster writes "Dear /.ers (slash-daughters?) I am looking for an elegant online solution to a real-world annoyance. I lead regular tours of my local cemetery, which I enjoy immensely. However, I hate the administration - can you recommend an on-line service, or some software (ideally open source) that I could host on my website, that would allow people to register for these tours, perhaps issue them with a ticket, and even let them cancel their reservation, all without allowing the total number of reservations for each event becoming more than, say, 25? Thanks."