Not sure what type of event you have in mind, but you might start by looking around the Active.com site - they've been in the event registration & marketing business for a long time and although they clearly have a service to sell, they work with some large successful events. Here's a link to get you started: http://www.active.com/race-director-software. I was an early adopter of active.com when I was a race promoter over a decade ago and the whole web service/e-commerce world was taking off, so it's been great to see them continue on.
