Recently Amazon released new beta service called Elastic Compute Cloud or EC2 . EC2 allows instantly starting (within 10 min) and running virtual Linux servers and paying on per hour bases ($0.10 per instance-hour consumed and $0.20 per GB traffic). For example running 10 servers during 1 hour or 1 server 10 hours will cost the same $2. Running one server all month will cost $72 + consumed traffic. Each instance predictably provides the equivalent of a system with a 1.7Ghz Xeon CPU, 1.75GB of RAM, 160GB of local disk, and 250Mb/s of network bandwidth.
EC2 makes possible to build highly scalable architecture easily and inexpensively. For example when load increases then EC2 customers can add more servers to the cluster for load balancing. Similarly when load decreases, these servers can be shut stop in order not to pay extra.
monitor.us can help you manage the capacity needs. When you see the site performance degradation, for example during rush hours, you can add more computing capacity only during these hours and then roll back to your normal configuration.
Please leave your comments if you know other or better scenarios of using EC2. I think it really may open some new interesting opportunities.