What are some awesome dedicated tools for running a subscription-based business?

Hey everyone!

I'm working on a fun post about FOCUSED tools for efficiently managing subscription-based businesses. Essentially, companies/tools that do one thing really well and dive really deep into solving a niche issue.

Postmark is a good example of what I'm looking for. They do transactional emails better than anyone else. It's their focused specialty. Other examples would be Chart Mogul or Cash Notify. Looking for dedicated, focused tools rather than all-in-one bundled services.

I wanted to open a discussion to gather some other ideas and allow you guys to drop a link if you think your tool fits the bill!

Thanks :)