https://hubdesk.io/. Create Github issues from emails and respond to them by starting a comment with "@reply". If you've got a side project, already use Github issues, and want to manage support emails the same as bugs. Easier and cheaper than signing up for a help desk SaaS.
At the moment my costs are about $100/mo, so it only takes a handful of subscribers to keep it going. Beyond that it was something I wanted for myself and I'm just glad if other people find it useful. I also learned a ton from making it and have been able to carry that over to my day job.
