Ghost
Ghost is open source and run by a nonprofit. You could, in theory, install Ghost on your own server, though most people opt to pay Ghost instead, including several former Substack publishers. Ghost offers an official guide for migrating and even a free concierge service that will handle the migration for you.
Buttondown
Buttondown is run by one person, Justin Duke, who responds to emails with questions basically instantly. It is bootstrapped, meaning there are no investors pushing for growth. We've talked about how to migrate your newsletter from Substack to Buttondown before, or you can email Duke and he'll do the migration for you (although we’d recommend giving the one man behind the one-man show a break and doing it yourself, it’s not difficult).
Beehiiv
Beehiiv offers one of the most generous free versions of any application on this list, supporting up to 2,500 subscribers, and offers a straightforward migration tool. There's also a referral program, meaning you can give discounts to readers who convince their friends to sign up
Revue
Revue is owned by Twitter, and setting up the service adds a big “newsletter” section to your Twitter profile. Now, it’s natural for any writer paying attention to be skeptical of Twitter, the company, but if you have a large audience on the social network, you might benefit from this integration. Revue offers a tutorial for migrating from other platforms, including Substack, but does not offer any kind of specialized service or tool for the job.