In researching topics for RailsCasts I often read code in Rails and other gems. This is a great exercise to do. Not only will you pick up some coding tips, but it can help you better understand what makes code readable.
A common practice to organize code in gems is to divide it into modules. When this is done extensively I find it becomes very difficult to read. Before I explain further, a quick detour on instance_eval.
You can find instance_eval used in many DSLs: from routes to state machines. Here's an example from Thinking Sphinx.
class Article < ActiveRecord::Base