you can use sinatra in place of rails for dynamic sites, but you'd need to pick template language (erb, haml), object relational model (activerecord, sequel, datamapper), and other things that come bundled in rails. that's what makes sinatra lightweight, you add as you need as opposed to everything there for you out of the gate. you can think of sinatra as being a simple domain specific language for creating sites or apis (i use it for both).

http://github.com/cloudfoundry/defekts