A naive Bayesian classifier to determine which Ask Metafilter questions a user should answer, depending on where, of the questions they've answered before, their answers have received the most favorites
Remember, the more questions a user has answered before, the more accurate this will be!
This is the single best explanation of Bayes' Theorem I know of, incidentally - http://yudkowsky.net/rational/bayes
And if you're interested in the history of Bayes' Theorem - http://lesswrong.com/lw/774/a_history_of_bayes_theorem/
Gems needed to run the damn thing (install them, then run "ruby kenobi.rb"): stemmer nokogiri mechanize