Fugue is a recapitulation and elaboration on the Prelude. It reexports most of the Prelude, substituting more general functions where appropriate (using Data.Traversable's mapM instead of Prelude or Control.Monad's, for instance). Additionally, it exports a bunch of base library modules that always tend to have a needful function or two that didn't find its way into the Prelude.
The library is largely an experiment to see how much havoc a "more complete" Prelude might wreak.