Instead of ActiveRecord, there are several options. The biggest competitor (and my favorite) seems to be DataMapper , then Sequel , Mongoid , MongoMapper . There are many others, but these are relatively large players.
Instead of an ActionView (not Active ... Action ... confusing, I know), I didn't see anything. I saw many additions to it, but no complete replacements. I think this means that people are mostly happy with the ActionView. However, there are other templates available. The big player here is haml . Using HAML does not mean that you are not using ActionView. You are still using ActionView, only with a different template preprocessor.
Alternatives to RoR itself. Again, there are a few. The big players are Sinatra , Merb (which started in the early days of Rails by some people who disagreed with something or something in Rails, and then Rails learned something and again took a few pieces and again ... confusing! ) There is also Ramaze , but I know little about it.
The thing is, Rails is more or less unsurpassed in what it does. If you get access to a web application written in Ruby, it will probably be either a Rails application or a Sinatra application. Sinatra meets another need for Rails. Rails wants to provide you with a whole set of tools for quick solutions, while the main goal of Sinatra is to be light and stay out of your way. The rails are huge, while Sinatra basically compiles an illustrious map of routes / dispatchers (I mean it in a good way), leaving you to create a web application, but you consider it necessary. For example, Sinatra makes it easy to pack a small web-based admin tool inside a gem. You probably won't want to include the Rails app in the gem. That would be very difficult.
In terms of “other components,” the great meaning (though not the Rails thing) that comes to mind is Test :: Unit. Although Rails just comes with Test :: Unit and does not limit you to using something else, it's worth mentioning that there are other testing platforms. RSpec is another major player here, as well as ZenTest . You can also see Shoulda , which is an add-on, more than an alternative.