This tip probably isn't as unknown as most of the tips I plan to list here, but it's definitely the feature I use the most. And a lot of the shortcuts will work with programs in addition to Word, so I wanted to cover it first as one of the basics. :)
You can find a full list of keyboard shortcuts here, but I'll share a few that I find particularly useful and use frequently.
ctrl+z = undo This is probably the one I use the most. It's nice to know that no matter what you do to screw up your manuscript (or photo in PhotoShop--I love this shortcut for PS) you can undo it with a couple of keystrokes. And I love how close the "z" key is to the "ctrl" key.
ctrl+y = redo Nice if you undo something and change your mind. No retyping.
ctrl+a = select all This will select all text. Much easier than running through 250+ pages. Another one that super helpful in Photoshop.
ctrl+c = copy selected text
ctrl+v = paste copied text
ctrl+x = copy and cut selected text Saves you a step when you're moving things around.
ctrl+i = turn on/turn off italics
ctrl+b = turn on/turn off bold
ctrl+u = turn on/turn off underline
ctrl+s = save Quick way to save your document as you're writing so you don't lose anything. I know Word is supposed to do this automatically, but I've learned not to rely fully on what any program is supposed to do. Bad things happen when I do that. :)
The shortcuts listed here all work in Blogger Editor as well.
So, how many of you are using these already? Do you have any favorites I didn't mention?
More Word tips posts here.