Google Search: yplus 3.01: People, don’t expose stupid pages and let search engines index them. The results just confuse everyone.
I found a certain User-Agent in my log files about which I wanted to find out more information, so I Googled for a term in the string. All I got back were other people’s log files that they apparently felt were interesting enough to let Google index. They unintentionally spammed a search term, insofar as “spam” means “render useless by adding meaningless crap.”
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I think yplus is added to a browser's user-agent string if the Yahoo toolbar plugin has been installed.