Script Summary: Automatically log into most sites saved in the Password Manager
Copyright: 2008+ by Adam Katz
License: AGPL v3+
No more need to actually click the button, this FULLY automates the login (assuming you only have one account saved).
I've been using this very effectively for two months. It only got me in 'trouble' once, when I was trying to log into an interface as another user (solved by merely disabling greasemonkey for the login). Otherwise, the infinite loop problems discussed below have been fully solved by my anti-loop checks. Userscripts.org (e.g.) logs me in fully automatically.
Some notes: I'm not sure how stable this is, specifically regarding infinite loops. There are two anti-looping checks. One merely looks for words indicating a bad login attempt (or a log-out link), which means some pages won't auto-log-in while others will still loop infinitely. The other tries to ensure that the referring page is not also the current page, but I don't think that works as intended (actually, I think it ends up never activating). So it goes.
Known bug: Some sites bring you back to the login page when you log out. For most of those sites, this script will log you back in (d'oh!). Many of those sites fail to mention that you've successfully logged out, which means there's no way for me to implement a check to prevent this. The best solution I have is to use the add-on mentioned below.
In a sticky situation, either log out of the master password so that nothing gets filled in or temporarily disable Greasemonkey or this userscript. I also use Master Password Timeout to prevent auto-logins.
Hmm, looks like I may have re-invented the wheel...