Some other services not mentioned there that I've used for free recently:
- humio cloud for log aggregation
- nodequery for server monitoring (though its future is unclear)
- braintree used to waive a big chunk of payment fees ($50k?) for startups, but I'm not sure the program exists anymore
Beyond things that are literally free, I'm a big fan of cheap vps providers if your uptime can take it and you can do your own ops. One of my projects makes ~$200/month and is hosted for $6/year on a 256MB vps. Lowendtalk/Lowendbox is the usual place I find these.