We use a lot of services at DNSimple. Here are some of them:

Chargify (http://chargify.com/) - recurring billing

Stripe (http://stripe.com/) - payment processing

Librato (https://metrics.librato.com/) - metrics

Mailgun (http://mailgun.com/) - email service

Boundary (http://boundary.com/) - network analysis

Papertrail (http://papertrailapp.com/) - consolidated logging

PagerDuty (http://pagerduty.com/) - alerting

TravisCI Pro (http://travisci.com/) - continuous integration

CodeClimate (http://codeclimate.com) - static code analysis

BugSnag (https://bugsnag.com) - exception tracking

Also, other services that have already been mentioned (like Github, Intercom, Trello, Dropbox, Sendgrid, NewRelic, etc)