Alex Polvi gave me a demo of the newest release of Cloudkick last week. Cloudkick lets you manage and monitor server instances running on Amazon EC2 and other cloud providers using a single, unified interface.
Instances can be tagged, and the tags can be used to perform collective actions. Using a small, low-overhead agent installed on each instance, Cloudkick monitors CPU load, network bandwidth, and memory usage. The data collected by the agents can be graphed, with optional grouping by tag.
Cloudkick can record system events and configuration changes, and these can be used to annotate the graphs in order to make it easier to correlate changes to the system configuration with changes in performance or reliability.
The basic version of Cloudkick is free. Advanced features are available for an additional monthly fee based on the number of server instances under management.