Github - obviously
AWS - obviously
Ylastic - easier AWS management
Sendgrid - mail delivery
Stripe - payment processing
Pingdom - external uptime tracking
PagerDuty - ops alerting and scheduling
Xero - accounting
JIRA - task management (the hosted version at Atlassian)
Desk - support tickets
Crashplan - personal machine backups
Google Docs/Mail - everything else
Others have been mentioning Fabric, Puppet, Graphite, Nagios -- we use these but they're not hosted services, so not sure they fit.