One thought I had was ticketing services. There are lots of groups that need to sell tickets online, but would rather avoid having to pay the commissions charged by the usual ticketing companies. Eg. Fringe theatre companies & fringe venues, amdram societies, small music venues (I'm sure there are lots more). There would still be a cost in using a card payment service, of course, but the front end could run on the society's private server for free. All it needs is someone to write that free front end. Hmmm. ;-)
Are you thinking of an online event management / booking system, like EventBrite? If so, I happen to working on a project will be a Free Software alternative:
The plan is to offer a hosted solution as well as the source code for download (its licensed as AGPL). Check out the wiki for planned features. Still work in progress, but feel free to chip in your skills :)
If you had in mind a ticketing system that can be used at an actual venue, like a point of sale solution then maybe you'd be interested in Lemon POS: http://kde-apps.org/content/show.php/Lemon+POS?content=69945
Thanks,
Sam.