Here are a few that we use.

Airbrake (http://airbrake.io/) - Exception logging.

Campfire (http://campfirenow.com/) - Chat.

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

Mixpanel (http://mixpanel.com/) - Analytics, people tracking.

Pagerduty (http://www.pagerduty.com/) - Monitoring alerts.

Sendgrid (http://sendgrid.com/) - Sending emails.

Sprintly (https://sprint.ly/) - Project management.

Tarsnap (http://www.tarsnap.com/) - Offsite backups.

As well as all the obvious ones - GitHub, Google Apps, Dropbox, etc.