With so many to choose from, what are you guys using and why? We are using AWS. This is what we are thinking of using...<p>Scalr.net - auto scaling for bringing up/down EC2 instances. Customizable, scripting and cheap. Not sure if proven though.<p>Cloudkick - monitoring. Easy UI and free.<p>Mixpanel - analytics. On action type analytics makes it easy to see which features are used and which are not.