I have been using Amazon Web Services for some time now and decided to use the Open Source Scalr Project to manage my farms on AWS. After overcoming many hurtles to getting Scalr running successfully I have been using it to manage my farms for about a month. Compared to the initial outlay required my RightScale the time it took to get Scalr running was nominal. Plus I like the ability to have a developer tweak the functionality of Scalr to fit our business requirements. There is an active Google Group for Scalr that I have used to solve most of my issues. People also have the option of using Scalr.net as a pay per month solution to manage their AWS farms. I chose to host my own instance of Scalr since we are doing large scale hosting and the previously mentioned need to customize it. I do enjoy the ease Scalr provides in bundling new custom roles I build for our various application servers. It allows you to simply press a button to save a new role for future use. Along with its ability to auto-scale as traffic dictates those are the two biggest pluses for me in using Scalr.
I will be adding more on my experiences with Scalr in coming days. If you are installing on CentOS5 I have some install notes I posted here.