Another huge upgrade with again close to 100 changes, additions, and tweaks. Most importantly are the new faster development environment that caches the framework and lets the application reload under both WEBrick and Fast CGI. Major is also the new breakpoints that allows you to inspect an application during execution, change the model, and resume. Additionally, we got a whole new way to do validations that extends the domain language of Rails with a wide range of new words.
Changelogs: Rails | Active Record | Action Pack | Action Mailer
But more importantly, learn how to upgrade from 0.8.5 to 0.9.0 or check the book about 0.9 features