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The ia_archiver is one that has been around for a bit, and is particularly iritating; a couple of mentions here [webmasterworld.com] and here [webmasterworld.com]. Basically, [url=www.alexa.com]Alexa.com[/url] crawls the web on this user-agent and flogs off the results to [url=www.archive.org]Archive.org[/url], the so-called internet archive.