Today I discovered PureText+, a little gem of a tool.
I copy+paste text from one program to another several times a day. By default, this preserves the formatting of the copied text, and I get around this copy+pasting into a plain-text editor (my editor of choice is TextPad) as an intermediate step.
PureText+ skips this intermediate step and pastes "pure" text without formatting. You simply have to use Windows+V to paste instead of Ctrl+V.
Get it here: http://code.google.com/p/puretext-plus/
(It's open-source and written in C#)