In short, Riemann is a complex event processor, driven by Clojure configuration, oriented towards monitoring distributed software systems. There's an introduction to its features and concepts here:
http://aphyr.github.com/riemann/
You can watch me try to justify my crimes against computers here:
And here's some slides!
If there's any questions I can answer, happy to help. You're also welcome to stop by #riemann on freenode. Cheers!
"Riemann aggregates events from your servers and applications with a powerful stream processing language. Send an email for every exception raised by your code. Track the latency distribution of your web app. See the top processes on any host, by memory and CPU. Combine statistics from every Riak node in your cluster and forward to Graphite. Send alerts when a key process fails to check in. Know how many users signed up right this second."