Key servers are repositories of keys in public key systems, they are necessary infrastructure for the widespread and easy use of systems such as PGP and GnuPG. KeyServers are publically accessible repositories of keys which alow for querying, retrevial and long-term archiving of public keys.
There are many lists of publically accessible key servers, but many of the are very out-of-date. At the time of writing 
http://keyserver.kjsl.com/jharris/keyserver.html was a good, current list.
(I use and recommend the mit KeyServer: hkp://pgp.mit.edu / http://pgp.mit.edu/ StuartYeates)
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