Toolbar icons in some programs (Mail, Safari, Preview, Disk Utility, but not in Finder) have a contextual menu item called Keep Item Visible -- Control-click on any icon in those programs to see it. This choice makes the selected icon, obviously, visible even when resizing the containing window. In other words, items on the left will disappear to make space for the one(s) you like -- pretty neat.
I haven't found this documented in Mac Help ... and it's a shame it doesn't work in the Finder, so the praised Spotlight search box disappears with smaller windows.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050513154002104