This passed the vote on the libraries list a while ago, but I forget to add a ticket. The request was to add sortOn to Data.List:

-- | Sort a list using a key on each element.  This implements the
--   decorate-sort-undecorate paradigm, also called a Schwarzian transform.
sortOn :: Ord b => (a -> b) -> [a] -> [a]
sortOn f = map snd . sortBy (comparing fst) . map (\x -> (f x, x))