If you are in a Cocoa application and typing along, but you just don't feel like typing the rest of the word you are in the middle of just hit the 'Esc' key. A menu with a bunch of different possible completions for the word you started are offered up to you in a nice, scrollable interface. Simply click on the one you want to go with and let the OS do the typing.
Now, this is pretty cool though I question the real world usefulness of this application. However, anything that you can show to a PC user to impress them about Tiger is good enough for me.
Tip of the hat to michael for sending this my way.