This happened to us so many times while building Google Chrome that we added a special shortcut to do just that. Alongside the common Ctrl-V keyboard shortcut for "paste", Google Chrome supports a similar shortcut, Ctrl-Shift-V, for "paste as plain text". (And it’s Command-Shift-Option-V on a Mac.)
You can use this shortcut in any rich text editor (like Gmail's compose window, or when writing in Google Docs) to strip out all the presentation from the original source and just paste in a block of text.
