I'm amidst a transition to cloud services. Here are some of the services I'm using right now
Great call quality, flexible call routing, and no more running our own PBX. Deleting the Asterisk VMs was a joyous feeling.
Similar to Google's offerings, but the email has better compliance features, and the online document app can handle complex spreadsheets more reliably than Google Docs.
LastPass makes it easy to practice good password hygiene, and it supports two-factor authentication for added security.
My accountants prefer Quickbooks to Xero and the Online edition makes it easy to collaborate at a distance.
HelloFax is a combination fax service and digital signature service. I'm much happier with HelloFax than I ever was with eFax.
I'm using Route 53 for DNS, and Glacier for archiving. I want to like EC2, but most of my workloads don't play well with the platform's constraints, and most of EC2's benefits don't provide a lot of value to my current workloads. As it stands, it would just triple my TCO.
This is an incredibly effective way to keep notes and documents available across multiple platforms and to share them with others. Evernote Search has helped me quickly identify relevant documents more times than I can count.
PagerDuty makes on-call scheduling incredibly simple. It's orders of magnitude better than Nagios's built-in capabilities.