Hexagonal Architecture

http://martinfowler.com/articles/badri-hexagonal/

http://alistair.cockburn.us/Hexagonal+architecture

vs. Layers

http://silkandspinach.net/2005/03/22/the-middle-hexagon-should-be-independent-of-the-adapters/

http://matteo.vaccari.name/blog/the-hexagonal-architecture