The project I'm working on (http://www.musubimail.com) was originally meant to help keep in touch with professional contacts, but I've been using it for beta-user follow ups to some good success.

I made groups based on how soon I need to follow up (this week, next quarter, etc) and have them track certain labels in my Gmail inbox. If someone needs to move groups, I just flag their email with the right label and wait for the reminders.

Let me know if this is something that interests you!