You can download Mongrel here. This is a project by Zed Shaw that is interesting: like WEBrick, but with some low level C code and considerably better performance running servlets.
While I think that Rails is great stuff, my interest in Ruby is driven more by the Ruby language itself. I am working on an Enterprise Ruby book right now and only about 20% of the book is about Rails and the rest covers 'Ruby coolness' in general. I continue to be amazed at how quickly non-trivial programs can be written and debugged in Ruby.
Anyway, good job Zed - I really like WEBrick because it is light weight and an simple platform to program to for all sorts of services and Mongrel looks like another tool that I will use a lot.