I have a project that started as a request for an online registration form, but has now morphed into a typical full conference web site: Schedule, presentations, registration with online payment including workshops excursions and all the other goodies that go with an international academic conference.
Rather than reinvent any wheels, I thought I'd turn to the smart people on the forum here and see what you use. Do you have an external SaaS that you use to handle conference registration sites, or have you built an internal system? Which services or tools do you use? Your advice and experience will be a big help in tackling this one!
If you are going to be doing a lot of conferences, events, registrations, etc. I'd suggest looking into Wordpress plugin called Event Espresso. It's a very robust event registration and ticketing system that has a lot of payment gateways included, with the ability to develop your own if you need one they don't offer currently.
I'm currently setting it up on a test server and getting it set for our speakers, student life events, etc. and it's not too bad to get going. Working on a custom payment gateway at the moment, which I think is what will take me the longest.