StackTach is a debugging / monitoring utility for OpenStack ([Open]StackTach[ometer]). StackTach can work with multiple datacenters including multi-cell deployments.
Watch the video here: http://www.youtube.com/watch?v=pZgwDHZ3wm0
OpenStack has the ability to publish notifications to a RabbitMQ exchange as they occur. So, rather than pouring through reams of logs across multiple servers, you can now watch requests travel through the system from a single location.
A detailed description of the notifications published by OpenStack is available here