Daring Fireball: Can I Get an "Hallelujah" for Auto-Completion With the Esc Key?. Hit F5, get an auto-complete drop down. Works in Nisus Writer Express. In 10.4 the Esc key does the same thing. Who knew? It raises the old usability question -- how do users learn about these occult features? What happens when one runs out of keyboard bindings (on laptops F5 controls the volume, you have to type Fn-F5 to get this).
Practically speaking auto-complete is extremely important for pen text entry, but it's far less useful for keyboard text entry. As DF notes it might help when one is unsure how to spell a word.