The Changelog

import System.Environment
import Data.List
import Data.Char
import qualified Data.Map as Map

custwords = filter (/= "") . lines . map (conv . toLower)
    where iswordchar x = isAlphaNum x && isAscii x
          conv x = if iswordchar x then x else '\n'

wordfreq inp = Map.toList $ foldl' updmap (Map.empty::Map.Map String Int) inp
    where updmap nm word = case Map.lookup word nm of
                             Nothing -> Map.insert word 1 nm
                             Just x -> (Map.insert word $! x + 1) nm

freqsort (w1, c1) (w2, c2) = if c1 == c2
                                 then compare w1 w2
                                 else compare c2 c1

showit (word, count) = show count ++ " " ++ word
main = do args <- getArgs
          interact $ unlines . map showit . take (read . head $ args) .
                     sortBy freqsort . wordfreq . custwords