"Peer-to-peer architecture embodies one of the key technical concepts
of the internet, described in the first internet Request for Comments,
"RFC 1, Host Software"  dated 7 April 1969. [...] The earliest
peer-to-peer network in widespread use were the Usenet news servers,
which communicated with one another as peers, propagating Usenet news
articles over the entire Usenet network, which particularly in the
earlier days of Usenet also used UUCP to extend even beyond the
Internet. However, these news servers also acted as client-server when
the individual users accessed the local news server to read and post
articles." (SOURCE: Wikipedia, Peer to Peer,
Napster was a file-sharing network which was launched in 1999 and
partly used a peer-to-peer architecture. It was developed by Shawn
Fanning. (See: Wikipedia, Napster,
A full peer-to-peer architecture was used by Gnutella and eDonkey,
both launched a few month later, in early 2000. Gnutella was developed
by Justin Frankel and Tom Pepper
(http://en.wikipedia.org/wiki/Gnutella); eDonkey by Jed McCaleb.
Kazaa was launched in 2001, and was developed by Niklas Zennström and
Janus Friis (who also invented Skype); eMule was launched in 2002 by
Hendrik Breitkreuz, who developed it as an open soruce solution.
I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it.