I can't remember what I was originally looking for when I found these Safari bookmarklets on the Apple site: Scriptable Applications: Safari Bookmarks.

Down toward the bottom of the page are two very useful ones -- one that resizes the browser window to 800x600 and one that makes it fullscreen. I'm working on something at work that has to be 800x600, so I'm finding that first one very useful in testing.

Oh yeah--this is what I was originally looking for: Safari Buttons. This add-on replaces the "Bug" button with a "New Tab" button, which I use constantly.

And this one gives you a warning when you attempt to close Safari if you have more than one tab (or window) open. Another very useful one, since I do that all the time.