You can access previous pages you've viewed in the current window by using the Back and Forward buttons in the toolbar. Speed up this look-at-every-page approach by pressing (clicking and holding) either button instead of just clicking it. You'll see a menu that lists all the sites in each "direction," so you can go directly to any page.

If you remember the beginning of the address for a previously viewed page, start typing it in Safari's address field (press Command-L to activate the field). Safari will fill in the URL for the most likely match, changing its suggestions as you continue typing. Start with the name of the site--you don't need the leading http://. For instance, when I type da in the field, Safari suggests daringfireball.net.

As a bonus, you get a drop-down menu from the field with other matches based on your history and bookmarks.