- ssh-add (allows one to automatically authenticate ssh connections): There is no need to use ssh-agent any more. If your key is called ~/my/key/file then ssh-add ~/my/key/file does the trick and globally enables automatic authentication (even for GUI applications!). If your key file resides at .ssh/id_pub then you can even omit the argument.
- Starting X11: There is no need to change the DISPLAY environment variable or to start X11 manually, any more. Instead, X11 starts automatically as soon as one invokes an application that uses X11.
Leopard: Things you don't need to fix any more
There are two things that are very simple under Leopard that used to be a lot more complicated: