One of my favorite "open source architecture" essays is on Graphite:
http://www.aosabook.org/en/graphite.html
It's part of the book, "Architecture of Open Source Applications", which has many such essays. This one is freely available -- and quite good.
Graphite is used for the business purpose of simple & fast real-time analytics for custom metrics inside an organization. It was built inside Orbitz and is now widely used at many startups, including my own.
Graphite is now a vibrant open source project with a community around it here: