LogMeOutThx is the fastest way to log you out of websites. Get it here or get it from the Chrome Web Store.
The most recent version is the Chrome extension but you can get a bookmarklet version from logmeoutthx.com.
The script looks for elements in the page that look like "Sign off", "Log out", etc. and dispatches a click event on them. If no such element is found, well, that's too bad.
This is provided under the MIT License.
(c) 2013 Timothee Boucher