Recurly for recurring billing (http://recurly.com/) (they just added support for VAT, ideal for us in the UK, and they support SagePay UK as a payment gateway as well as PayPal)

Unfuddle (http://unfuddle.com/) and github (http://github.com/) for bug tracking.

JungleDisk (http://www.jungledisk.com/) for off-site backup. Uses your S3 account to store encrypted backups.

Server Density (http://www.serverdensity.com/) for monitoring

UserVoice (http://uservoice.com/) for customer feedback