For my solo SaaS company:

Backups:

- Backblaze (disaster recovery for my laptop)

Hosting:

- Cloudflare Workers

- fly.io (actually free when under $5/mo usage but I wish I was giving them money)

- AWS

Emails:

- Postmark After messing around with emails that weren't being received (especially with email-only auth), glad I found Postmark

- Gsuite

- Dmarc Reports Monitoring to see if I configured Dmarc correctly everywhere