Serve is a small Rack-based web server and rapid prototyping framework for Web applications (specifically Rails apps). Serve is meant to be a lightweight version of the Views part of the Rails MVC. This makes Serve an ideal framework for prototyping Rails applications or creating simple websites. Serve has full support for Rails-style partials and layouts.
It's possible that someone on another system could have Thin fail to install but Mongrel install. For this reason, I think that for this type of project it would make sense to allow for more than one web server.
Mongrel didn't install on OS X. Rather than go through the trouble to figure out why, I installed thin, and made serve fall back to it.
http://github.com/benatkin/serve/commit/c0c2d240d93fad2e016ab940507ae4f60be80810
It's possible that someone on another system could have Thin fail to install but Mongrel install. For this reason, I think that for this type of project it would make sense to allow for more than one web server.