Exploring modular web applications with Sinatra.
If the app has fairly unique content per page, Sinatra works really well actually. (Mostly because it's a thin layer over Rack.) Sharing anything though, basically means implementing a module or base class and implementing it everywhere.
An added benefit is that these applications could be used in Rails apps as well. I'd probably think hard about this, because Rails tends to be a resource hog.
Here's an example config.ru I used for running against the dev system: