Appending things is used as an example a lot of the time so people sort of get it into their heads that monoid = concatenation, which is obviously not true. I can't speak as to which monoids get used the most in Haskell but I have personally used it for appending string-like things, and also to unify data structures (combine the stuff in them basically). There is actually a library that will derive a Monoid instance for you if all of the types you're using are already instances.

http://hackage.haskell.org/package/generic-deriving-1.6.2/do...