Corsizio has a host of great options including taking payments and iframe integrations for embedding registration forms directly on your website. The registration forms can also be customized and all the data can be exported at any time. Integrations with MailChimp is also key. Even more so is that if you have under 50 registrations per month it's free, so it's a great platform to build on when you're starting out. And when you need to pay chances are you'll have built up your business to have it pay for the premium fees which are pretty nominal. Review collected by and hosted on G2.com.

They use Stripe for payment processing and I wish they also worked with Square Review collected by and hosted on G2.com.