UserStamp: Automatic created_by / updated_by for Rails models
Delynn Berry has developed a useful plugin called UserStamp. Whereas Rails has built in support for automatic columns such as created_at and updated_at, it doesn't (quite rightly) support concepts such as created_by, as these belong in the application domain. It's a commonly required feature though, and Delynn has done a great job of packaging it up.
This is also worth looking at if you want to get an idea of how to put a similar plugin together. Most Rails applications have similar requirements and packaging them up as plugins makes it a lot easier to manage.