Ruby Monk is a fun, interactive overview of the Ruby language. Each topic has a short explanation followed by a code example and a quick exercise. I find that it is especially helpful as a quick review for learners who have seen the concepts previously in other contexts. The best part about this resource is that the code exercises run in the browser, right there beneath the explanatory text. There's no need to open up a terminal, allowing you to fly through this comprehensive review.
Ruby Symbols Explained: If you can get past the aesthetics of the site, this is an exceptional explanation of Ruby symbols. It shows the difference between strings and symbols, and when it is appropriate to use each.
Floating Point and Currency: "Do Not Use Floating Point for Currency." The first sentence of this blog post sums up it's major take-away. If you'd like to understand why this is such sage advice, I highly recommend reading this short article. While it's not a comprehensive guide on the subject, it's a quick read, and should leave you understanding why 0.1 + 0.02 is not equal to 0.12 in the world of Ruby.
Agile Development with Rails: The vast majority of Ruby developers use the Rails framework, so once you have built a basic understanding of Ruby, it would serve you well to learn Rails. If you've already completed Hartl's Rails Tutorial, the next step is to read Agile Development with Rails. Like Hartl's Tutorial, it will walk you through building a simple web application. The key difference is that it also explains how Rails works behind the scenes. You'll get an excellent introduction to Rails conventions and the gems that make up its backbone. The Rails 3.2 version is available free online, but if you want to learn with the current version of Rails, it's worth buying a copy of the most recent version of the book.
Rails Guides: Ready to start building your own Rails apps from scratch? You're going to need a hand from the official Rails Guides. This set of guides includes many topics, from setting up a basic rails app to running migrations, and even keeping your application secure. It can be read straight through, but it's also perfect as a reference to look up the correct syntax for running a migration, creating a model, or specifying a route. Even highly experienced Rails developers visit these guides often.