The Cmd key also acts like Alt for Cmd-Tabbing through open applications. Cmd-` (key above tab) for tabbing through open windows within an application.
Cmd-Z is usually Undo, but it depends on the app. Cmd-(Up) and Cmd-(Down) act as PgUp and PgDn in most apps too.
I really do find that Cmd is much more conveniently placed than Ctrl for common and/or repetitive key combos. On a related note, in the "Modifier Keys" section of the "Keyboard" System Preferences, you can change the function of all of the modifier keys if you happen to like your Ctrl key better than the Cmd key. You can also turn the Caps Lock key into a Ctrl key (or Cmd, or Opt) for the old school UNIX guys. Apparently this makes Emacs much more usable, but I'm a vim snob, so I wouldn't know.