Hullo! Thought I'd post a non-LJ Nifty here =) I've known about this for years and it's never ceased to amaze people when I tell them.
- In MS Word, open up a document that you already have going.
- Put your cursor somewhere top-left-ish.
- Hold down SHIFT and ALT, then click somewhere bottom-right-ish.
- Drag your mouse around a little to see what happens =)
- Stop somewhere, copy and paste and see what happens.
I have other Word and Excel nifties too, but I'll let other people have a turn first =)