Our helpdesk app, both the SaaS and the self-hosted version - is powered by SQL Server. Of course, the SaaS app additionally uses S3 (to store file-attachments), Redis (to persist in-memory cache between deploys and restarts) and other fancy cloud stuff, but there's still one big ass SQL Server database at the heart of it.
And this Christmas we've migrated it to Linux.
After the recent Facebook outage I got curious how many SaaS B2B products have an actual SLA - a Service Level Agreement with a legally binding uptime commitment. Turns out SLA's are very uncommon in the SaaS space. Here's a little comparison table I compiled, just for illustration purpose.