Better Errors is a Ruby gem that replaces the standard Rails error page. It's incredibly easy to use and helps both functionally and aesthetically.
This gem is only for development, so to get started add it to your Gemfile.
Though Better Errors works without it, I would personally recommend adding the Binding of Caller gem as well. It gives grabs bindings from further up the call stack, giving you better and more specific errors.
One of the nicest parts of this gem is that once you added it to your Gemfile and run
bundle install, that's all you need to do. There's more you need to do if you aren't using Rails, but even that isn't much.