I've lately found myself frequently in a situation where I had a directory structure that I wanted to be able to easily browse like a server. One common reason for me was a desire to browse work I was doing on my Mac on Parallels.
Rails uses Webrick to quickly mount its directory structure, and since my needs were very lightweight, I decided to investigate how I could set up webrick to mount a directory structure on a particular port. The result: a small script called server.rb which you can drop into any folder and call via
ruby server.rb [PORT]. It'll default to port 2000.
The code is almost entirely the example code from the Webrick library, but there are a few modification. Bon Apetit!