¶Distributed systems » Peer-to-peer
A peer-to-peer system is a distributed system where all of the nodes in the system have (or are eligible to have) the same responsibilities. This differs, for example, from a client/server architecture, where there is a clear difference in behavior between client nodes and server nodes.
My personal favorite peer-to-peer system is Pastry. I was first introduced to it by Jeff Janies when I worked at RedJack.