Mongrel is a small library that provides a very fast HTTP 1.1 server for Ruby web applications. It is not particular to any framework, and is intended to be just enough to get a web application running behind a more complete and robust web server.
What makes Mongrel so fast is the careful use of an Ragel extension to provide fast, accurate HTTP 1.1 protocol parsing. This makes the server scream without too many portability issues.
There are also many more new options for configuring the rails runner including changing to a different directory, adding more MIME types, and setting processor threads and timeouts.
It doesn't explicitly require Camping, but if you want to run the examples/camping/ examples then you'll need to install Camping 1.2 at least (and redcloth I think). These are all available from RubyGems.
If you run this and access port 3000 with a browser it will say "hello!". If you access it with any url other than "/test" it will give a simple 404. Check out the Mongrel::Error404Handler for a basic way to give a more complex 404 message.
This also shows the DirHandler with directory listings. This is still rough but it should work for basic hosting. *File extension to mime type mapping is missing though.*