Mongrel: A Web server written in Ruby.
Mongrel is a fast HTTP library and server for Ruby that is intended for hosting Ruby web applications of any kind using plain HTTP rather than FastCGI or SCGI. It is framework agnostic and already supports Ruby On Rails, Og Nitro, and Camping frameworks.
Writing simple servers is actually pretty easy in Ruby. I remember a sample in the Pickaxe book which demonstrated a Web server in about 15 lines of code.