Rack::LayoutWrapper makes it super-easy to have shared layouts between a number of Rack applications – it’s as simple as using Rack::LayoutWrapper, setting the template dir option and then using the passed in layout object (as a part of the rack env).

In the same vein as Rack::URLMap or drnic’s probably versioned submission, API Version Mapper provides
a simple way to declare and namespace different api versions for a Rack app.

A configurable (yet still simple) set of middleware that makes it easy to limit user requests (typically for an api)
based on a # / hour limit. Combined with other middleware it’d be easy to do routing.