Haskell Indexer is a Kythe extension for working with Haskell source code. Kythe is language-agnostic ecosystem for building tools that work with code. An example is code search with cross-reference support: https://cs.chromium.org/. Haskell Indexer makes it possible to use Kythe-based tools with Haskell. With Haskell Indexer it’s possible to list all use-sites of any given function (get reverse-references) and explore the code without any IDE setup.
A portion of GHC and Stackage is indexed and available at http://stuff.codereview.me/.