Phusion Passenger is an Apache module that allows you to run Ruby on Rails applications from within Apache. It promises a minimal configuration experience beyond uploading your application files. I have not played around with it yet myself, but it looks interesting:
"Phusion Passenger" - http://www.modrails.com/
The name of the project seems to have gone through a few iterations, hence the modrails.com domain above. Previously it was known as mod_rails and mod_rack.
When I last looked seriously at Ruby on Rails, "Mongrel" - http://rubygems.org/gems/mongrel was the web server of choice but it would appear that Phusion Passenger has more traction now, especially for production environments. I found a good overview of the benefits of using Phusion Passenger versus Mongrel here:
"Replacing Mongrel with Passenger (mod_rails)" - http://davedupre.com/2009/03/23/replacing-mongrel-with-passenger-mod_rails/
Updated 2022 : note that the above post was originally published in 2010, but is left here for archival purposes. It is completely out-of-date, therefore I deactivated the external links, most of which are now broken.