What gems do you use for helping you be more productive or debug more easily while coding in Rails?
Some gems are specific to a task, and you find them easily when you need to implement the feature. With this question, I'm looking for all the others, that you might not think to search for, but once you have heard of it, you install it on each new project.
Please keep one gem per reply, to help the votes sort them out.
For me one of the most useful Rails debug tool is pry. This is something like more feature rich IRB console (we have access to all models, etc) and debugger - we might set break-points, watch variables.
rails-footnotes: displays footnotes in your application for easy debugging, such as sessions, request parameters, cookies, filter chain, routes, queries, etc.
Even more, it contains links to open files directly in your editor including your backtrace lines.
awesome_print: when in the Console, pretty prints Ruby objects in full color exposing their internal structure with proper indentation.
mailcatcher: catches email messages sent and display them in a web interface.
Run mailcatcher, set your favourite app to deliver to smtp://127.0.0.1:1025 instead of your default SMTP server, then check out http://127.0.0.1:1080 to see the mail that's arrived so far.
No need to pollute your inbox and wait for each mail to jump around smtp servers before reaching you. Must have when you are coding without an internet connection.
