I discovered this one by accident ... you can press Escape in the middle of typing a word in Cocoa applications (tested in TextEdit and iChat), and a list drops down with possible completions for what you typed.
[robg adds: We pointed this out in 10.3, but I'm re-running it now because (a) I don't know how many people know about it from 10.3, and (b) the keyboard shortcut has gotten simpler -- you used to have to hit Option-Escape. Now just a simple Escape brings up the list.]

