I saw this the link other day via a Hacker News link:
Michael Schurter works at Urban Airship. In this guide he goes through the process of fixing a bug in nginx (specifically, that HTTP PUT requests don’t accept empty bodies). It’s an interesting guide because it shows what tool you need to debug nginx C code.
In general, anything you can do to offload web requests from your app server to nginx will speed up your web pages signficantly.