So far I've started using Super User, Stack Overflow, Ask Ubuntu and Photography on the Stack Exchange network. Every time I log into one of them (with Google ID) and go to another one of these four sites, I automatically get a message on the top of the page saying I'm already logged in so all I need is to refresh the page and that's it. However, when I want to log out it's a whole different story. I have to enter each site and log out separately. Nothing seems to do it; I've tried going to stackexchange.com but logging out from there only logs me out of -you guessed it- stackexchange.com. Is there a way to do it?
An alternative answer: use a different browser profile. This is less impractical than it sounds: profiles are quick to create. This answer gives the relevant firefox command invocation:
Ideally stackexchange websites would provide a logout button that worked, of course.
However: if you close all tabs visiting stackexchange websites, and all tabs visiting relevant authentication websites (e.g. myopenid, google websites), and then delete all the relevant persistent browser state, that will do it reliably.
Relevant state consists of cookies, and HTML 5 local storage (stackauth.com). There are other types of other persistent state also (notably flash local storage), but I don't think stackexchange uses that other state. Most browsers have a search box somewhere into which you can type "stack" and see all the relevant state and quickly delete it.
There are likely extensions for your browser that will do that for you with fewer clicks.