Rails 3.1.0 Beta Engine Example - personal playground - DOES NOT WORK WITH Rails 3.0
Far from finished, but upto now it works nicely.

All functionality is extracted into modules and can get mixed into any object, which makes it easy to
enhance/reuse existing objects (models were my main requirement, but i thought its a good idea to
modularize from the beginning..

Comes with Rspec and an in-memory sqlite3 database, which is quite speedy (my main application went
from 9 to under 3 minutes).
But it doesnt seem to play nice with database_cleaner, as it's still recreating the database after
each step, even with enabled transactional database cleaning, but right now i dont care, still speedy.

Beware that my targetted use for engines is not 'separation', but 'combination', so this codebase isn't probably
for everyone, as an isolated_namespace is way easier to handle.

Use/abuse at your own discretion.

Usage is simple, just clone and include:
gem "imp", :path => "<path_on_your_filesystem>"
into your rails Gemfile, adjust routes and run generators (the latter is optional).

The spec/dummy directory was created using Jose Valim's enginex generator, thanks to martinstreicher for bringing up the question.