App Concerns are awesome.
They've been used for years by 37signals, but there's built-in support for them in Rails 3. They're basically a really easy way to DRY up your code, by letting you easily extend your models with a few handy extras like smart loading. Most Rails 3 gems (like Mongoid v2.0, for instance) use App Concerns to organize their code. Not only is this design pattern great for reusing code across models, but I also use it to refactor large models into component pieces for better readability.
Here's a basic example I was showing to a friend who is a new programmer (pardon some of the obvious OOP notes):
