This is one of the important peer-to-peer papers. My only criticism is that Chord only provides probabilistic guarantees. I don't see how it can be used as a building block for distributed systems that need to provide strong semantics.
I. Stoica, R. Morris, D. Karger, M. F. Kaashoek, and H. Balakrishnan. Chord: A Scalable Peer-to-peer Lookup Service for Internet Applications. In Proceedings of SIGCOMM 2001, San Deigo, CA, August 2001.