The Heroku Toolbelt is a package of the Heroku CLI, Foreman, and Git — all the tools you need to get started using Heroku at the command line. The Toolbelt is available as a native installer for OS X, Windows, and Debian/Ubuntu Linux.
The Toolbelt has been available since last fall as part of our polyglot platform. Since then it's matured substantially with a huge amount of user testing, and now even has a shiny new landing page. Ruby developers can continue to use gem install heroku, but developers in other languages (Python, Java, Clojure, etc) will probably prefer not to have to install Ruby and RubyGems to use Heroku.
The installer won't trample your existing install of Git if you have one. Similarly, although the Heroku CLI uses Ruby under the hood, the Toolbelt packaging isolates all of its libraries so it will not interfere with an existing Ruby setup.