Signupper is very similar to Unbounce and LeadPages. The difference is that those services are for landing and squeeze pages for marketers while Signupper is for signup forms for web applications.
The difference is somewhat nuanced and it is one of the things I am working on expressing clearly. If you have any pointers, I would be grateful!
A landing page builder like Unbounce is for hosting and managing a signup form on a landing page. The data is collected in an HTML form, some front-end validation is performed, and the data is pushed to some backend service. Most of the time, the data is sent to a mailing list manager like MailChimp, or a CRM tool like SalesForce. The backend service can not do any verification and present errors. While it is possible to send the data to an arbitrary web hook, the information flow is one way. The backend service cannot veto the data and re-display the form. This is OK as the worst that will happen is a contacts database or a mailing list will end up with dupe accounts. For web applications, that validation must happen.
Furthermore, it is a generally accepted practice that after filling out an account signup form for a web application, the user gets logged into their account seamlessly. It is annoying if, after signup up for a service, I am congratulated and presented with a log in screen. This is friction, and the point of conversion rate optimization is to reduce friction and improve end-user experience.
Signupper was built specifically to address the needs of a web application signup form. It is very much like Unbounce, but with the ability to host signup forms for web applications.
It grew out of a need that Unbounce and other services like it could not satisfy. I am now trying to find other companies who have that need and get a good feeling for what feature set is most important for them.
Looking forward for more questions, critique, and feedback.