Gimbal is the base Rails application I use when starting new projects. It was mostly copied from thoughtbot's Suspenders project, but modified for my needs.
Install the gimbal gem:
gem install gimbal
Then use it:
gimbal projectname
This will create a Rails app in projectname directory using the latest version of Rails.
You can optionally create a GitHub repository for your new Rails app. It requires that you have Hub on your system:
curl http://hub.github.com/standalone -sLo ~/bin/hub && chmod +x ~/bin/hub
gimbal app --github organisation/project
Which does the same thing as running:
hub create organisation/project