How do you log out of Stack Overflow? There isn't any logout button.
As the other answers say, the "log out" link is now hidden inside the "Stack Exchange" menu in the top
left right corner of the page.
Unfortunately, just clicking that link is generally not enough to log you fully out of Stack Overflow — if you don't also log out of your OpenID provider, you'll just be automatically logged back in when you next visit SO. (Yes, that happens even if you're using Stack Exchange to log in — the Stack Exchange OpenID provider is a separate system from the actual sites like SO.)
In practice, if you ever need to log out of SO (e.g. if you're using SO from a public computer), the easiest ways to do that are to either:
use the Private Browsing / Incognito mode in your browser when visiting Stack Overflow, or
if you've already made the mistake of logging in to SO outside Private Browsing mode, clear all cookies and other private data from the browser.
This is because Stack Overflow doesn't want us to leave, we're a lovely community fully of magic and you shall not leave us alone.