In Safari 4, Undo (Command-Z) simply undid the last action, such as typing, but not more general functions about the browser.
In Safari 5, Undo will reopen the last closed tab, or Undo the closing of the tab.
[crarko adds: I know this has been publicized elsewhere, but thought we should have it here for completeness. This hint was actually submitted by multiple people, and this one was first in the queue.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2010060915323366