Having trouble coming up with a word in Panther? Just type the first few letters of the word in question in a text input box (like the Safari URL bar or iChat's input area or TextEdit or ... you get the idea). Once you have a few characters, hit Option-Escape to see a pop-up list of possible completions...
I'm really not sure exactly how I might put this to regular use, but it is sorta cool ... and it makes for fun screenshots, hence the completely unrequired shot in this hint :).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031019214026732