Mailing lists
Ruby on Rails Talk is where Rails users come to seek help, announce projects, and discuss all kind of matters surrounding the framework and the community. If you're working on a patch, you can raise issues on the core list. We also have a dedicated list for Script.aculo.us and Prototype talk on Javascript and Ajax. Finally, there's the security announcement list for critical patches.
Twitter & Blogs
Follow along what people are saying about Rails on Twitter. It's a direct hook into the pulse of Rails. Or follow along in what people are blogging about Rails from Planet Ruby on Rails.
Wiki
The Rails wiki is the sprawling, weird, and wonderful pot of user contributions to the documentation and the Rails knowledge sphere. If you can't find something in the books or official guides, there's a good chance you can find it here.
IRC
Sometimes its just easier to get help or discuss matters in real time. The #rubyonrails channel on irc.freenode.net allows for just that. Try #rails-contrib if you want help or discuss your patch to Rails itself. Need an IRC client?