A while ago, I wrote a small package called servius, a simple executable that serves static files with Warp, and will additionally render Hamlet and Lucius templates. In some earlier package consolidation, the tool became part of shakespeare, and eventually was commented out (due to concerns around the dependency list on Hackage looking too big).
Today, I just resurrected this package, and added support for rendering Markdown files as well. I often times end up working on Markdown files (such as for this blog, the Haskell Documentation project, and the FP Complete blog), and being able to easily view the files in a browser is useful.
As it stands, the three specially-handled file types of Hamlet (.hamlet), Lucius (.lucius), and Markdown (.markdown and .md). If others wish to add more templating or markup languages to this list, I'm more than happy to access pull requests.
Final note: this package is currently uploaded using the pvp-bounds feature of Stack, so don't be surprised when the version bounds on Hackage are more restrictive than those in the repo itself.