Nice idea. I work for a company that manages hundreds of domain names and SSL certs. Our sales people have some sort of system for managing these now, but we often run in to issues with however they're doing it. I'll pass your site to my boss to see if he's interested.

One thing I'd like to see is the ability to create folders where I could name it the client name and keep all of their domains there. For example if xyz corp had xyzcorp.com, .net, and .org, I'd like to have all of those under one folder. I'd also like to see a dashboard that shows domains that are expiring within 30/60/90 days.