Also contains a benchmark (report.html) that shows that ordNub apparently is faster than nub in all cases.
PACKAGES_USING_NUB.txt contains all packages which use nub (made with a quick grep). It's not the most accurate since some packages define their own nub, but that's a minority.

This thing here is not a library. It is a benchmark suite. View results here.

Don't use nub

If you are looking for a fast ordNub function to use in your code, you can use:

import qualified Data.Set as Set

ordNub :: (Ord a) => [a] -> [a]
ordNub l = go Set.empty l
  where
    go _ [] = []
    go s (x:xs) = if x `Set.member` s then go s xs
                                      else x : go (Set.insert x s) xs

Other Data.List functions you NEVER want to use

\\, union, intersect