ascmaster is a simple Ruby gem for converting images to ASCII.
Unfortunately it has the worst dependency in the world: rmagick.
To install:
gem install ascmaster
Or, if you're using Bundler, add to your Gemfile:
gem 'ascmaster'
You can use ascmaster from the command-line. Just give it the path to a local image file:
ascmaster path/to/your/image.jpg
Enjoy!