I just released a new Drupal module. It was one that has been laying around for some time now, but got constantly improved to make it reusable on as much sites as possible. I stripped the feature set down to the minimum and called it "subsites".
Download it at its Drupal project page.
What are subsites?
Subsites are a part of your website that can have its own menu, theme, custom CSS or anything else you want.
An example? Suppose you are making a magazine website. On most magazine websites you might want to create small subsites about a certain topic. This subsite probably has its own menu (next to the primary navigation). It might also have its own look. This new look could be a minimal change from the default site look (a different background, different colors, ...) or it might be a completely new look.