Here are a couple roundups of services used by event organizers, from Idealware, a company that helps nonprofits learn about various software options (not just event-related). They include thoughts on how to choose a service as well as a summary of some popular choices. While aimed at nonprofits, most of the information would be useful to for-profits as well.
Registration systems (general admission):
Be sure to check the comments of the registration post – more services are mentioned.
Hope this is helpful.