UltraEdit has a nice feature in which when you press Ctrl-Space, an auto-complete list is opened, suggesting you to complete the word you just started to type. The nice thing is that its suggestions come not only from the list of keywords of the edited file’s language, but also from the actual words in the file (actually, the words above you current location).
This is great as you often use the same words several times in a file, and it can be handy for your editing tool to use that and suggest options from your current work, which probably knows the words in the domain more then any general keywords dictionary file.
I really wish someone would have written an add-in to Microsoft Word that does it. I might get to it some time when I have time (as if… ha).