A few that I swear by:

github http://github.com for source control and wikis

sendgrid http://sendgrid.com or Amazon SES for email

mogotest http://mogotest.com for browser testing

recurly http://recurly.com for managing recurring payments

tender http://tenderapp.com for support / help desk

linode http://linode.com for hosting or Amazon EC2

mocksup http://mocksup.com for quick and easy mockups

clicky http://getclicky.com for analytics

zerigo http://www.zerigo.com for dns and monitoring

AWS http://aws.amazon.com S3, EC2, and everything else :)