Monad is a monoid.

MonadMonoid.hs

class Monoid m where
    mappend :: m -> m -> m
    mempty :: m

mconcat :: Monoid m => [m] -> m
mconcat = foldr mappend mempty

newtype MonadMonoid m a =
    MonadMonoid { getMonad :: a -> m a }

instance Monad m => Monoid (MonadMonoid m a) where
    mappend f g = MonadMonoid (\a -> getMonad f a >>= getMonad g)
    mempty = MonadMonoid return


validations :: [MonadMonoid Maybe String]
validations = fmap MonadMonoid [
    \x -> if x == "Bad" then Nothing else Just x
  , \x -> if x == "Invalid" then Nothing else Just x
    ]

main :: IO ()
main = do
    print . getMonad (mconcat validations) $ "Bad"
    print . getMonad (mconcat validations) $ "Invalid"
    print . getMonad (mconcat validations) $ "Good!"

{-
  Nothing
  Nothing
  Just "Good!"
-}

You can also use Kleisli from Control.Arrow and generalize:

import Prelude hiding ((.), id)
import Control.Category
import Data.Monoid

newtype EndoCat c a = EndoCat (c a a)
instance Category c => Monoid (EndoCat c a) where
    mempty = EndoCat id
    EndoCat lhs `mappend` EndoCat rhs = EndoCat $ lhs . rhs

See also http://comonad.com/haskell/monoids/dist/doc/html/monoids/Data-Monoid-Categorical.html