Via Andrea Baruffaldi (with whom I coordinate the UML Lovers LinkedIn Group, feel free to join us!) I have discovered the UML Diagrams Examples site.
The site includes several examples of most UML diagrams. Even though there are some errors (same as when anybody publishes source code, there are always hidden bugs) I do believe the site is useful (given the basically zero repositories of UML models available online).
Remember that if you need to refresh your knowledge of the UML notation to understand the examples you can take a look at my list of UML cheatsheets and reference guides