Installing the heroku gem
I had some problems installing the heroku gem on Leopard:
$ sudo gem install heroku
Building native extensions. This could take a while...
ERROR: Error installing heroku:
heroku requires json_pure (>= 1.2.0, < 1.5.0, runtime)
So I tried to install the json_pure gem first (sudo gem install json_pure), but this always seemed to install/update the json gem instead of the json_pure gem.
I eventually managed to install the correct gem by providing the version number on installation:
sudo gem install json_pure -v 1.2.4
This installed the json_pure gem and I was also able to install the Heroku gem afterwards.