We use rack at personifi. Highly recommended simple elegant and fast!
Developing general web frameworks and frameworkless web applications is
difficult because lots of different ways to connect to web servers need to
be implemented. Since Ruby is popular for writing web applications, we try
to address this problem by proposing a lightweight abstraction mapping HTTP
requests onto a simple Ruby API. This makes it possible to combine all
kinds of web servers with different web applications without further
change. Furthermore, our solution enables new functionality by combining
and composing web applications, as well as better testability. Talk given
at the European Ruby Conference (Euruko) 2007.