A misspelling of "referrer" that managed to worm its way into the HTTP Request header standard1 (Nelson says to the IETF: Ha Ha!).
The misspelling has propagated2, and it is not uncommon to find documents that refer to the referer and the referred in the same sentence. For example: "The award will go to the referer who referred the most traffic for the previous month!"
The only time 'referer' should be used instead of referrer is when you are referring to the HTTP header (HTTP-REFERER) or to one of the programming language variables which refer to the Referer header. Examples of these include:
ASP's Request.ServerVariables("HTTP_REFERER")
Perl's $ENV('HTTP_REFERER')
1 HTTP/1.1 Header Field Definitions: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.36 (04/29/2002)
2 A Google search turned up 218,000 results for "referer" on 04/29/2002.
If you're keeping count, this w/u uses a form of "refer" 15 times. ;)