MS Office applications have special paste options, but accessing the “paste without formatting” option tends to take far too many mouse clicks. Also WordPress can be expanded via a plugin to provide a paste as plain text option. To get it to work in Firefox though, one must hack an obscure config file. To get around it, I’ve resorted to leaving Notepad++ open simply so I can paste into there first. This removes the formatting and I can then cut and paste to the true destination. It’s still a faff, but much less so that the previous options.
I have recently discovered a simple, free, application that effortlessly solves the problem. Pure Text sits unobtrusively in system tray and provides a new paste from clipboard option to all applications. By default the keyboard shortcut is Windows-P. Hit that key combination and it pastes the clipboard contents as plain text. Simple.
My only gripe is that it clutters the system tray, rather than being configured from the control panel. It’s a really minor irritation though compared with its usefulness.