Just to summarize the available options:
Tools
- PureText – free tool for Windows
- Use AutoHotkey and write your own macro
as suggested by Dean
Browsers
- To copy plain text from a browser: Copy As Plain Text or CopyPlainText (suggested by cori) – Firefox extensions
- To paste without formatting to a browser (Firefox/Chrome at least): CTRL+⇧ Shift+V on Windows/Linux, see below for Mac OS X.
Other
- Under Mac OS X, you can ⇧ Shift+⌥ Alt+⌘ Command+V to paste with the “current” format (Edit -> Paste and Match Style); or ⌘ Command+⇧ Shift+V to paste without formatting (by Kamafeather)
- Paste to Notepad (or other text editor), and then copy from Notepad and paste again
- For single-line text: paste to any non-rich text field (browser URL, textarea, search/find inputs, etc.)
Please feel free to edit/add new items