I should also mention DISQUS in the list of proprietary-but-gives-back-in-other-ways (like a great free tier) services I use
David Cramer's Sentry is an instance of a true open source company (using the SaaS model, where people pay for the hosted version). (Thought prompted by scanning the EuroPython schedule and seeing David's talk outline)
Loggly are another company that (like Sentry) focus on their hosted service, meaning they can open source logstash without needing to worry about monetizing deployments inside corporate firewalls.