You know how there's a while-you-type spell checker that's always on when you are editing text in a multi-line text box? There should be a feature that takes the last few words you're typing (or the entire current paragraph, or whatever works best), does a Google search, and presents snippets for the top few results in an unobtrusive pop-up window.
Sure, maybe you're thinking "It looks like you're writing a letter. Do you want me to write it for you?" (the Microsoft paperclip). But using web search instead of a fixed set of patterns could actually make this useful. Imagine the number of messages to customer support forums that will never have to be sent because this feature pops up the answer the user was looking for. And so on.
You might also think, "this already exists, it's called Google auto-suggest." But I specifically want it to work when I'm not (yet) actively searching, but just writing. (If it already existed, it might have stopped me from writing this blog post. :-) Twitter might also become a different place if users realized how many others have already entered the same item.
Of course there's a little privacy issue. But still, if this existed, I'd opt in! (In fact, I did half a dozen searches while I was typing this. How much easier it would be if I didn't have to select text, switch to a different tab, paste, and hit enter, losing my writing context each tim.)