If like me you use a lot of online services, you may want to use at least two different passwords depending on whether the site offers secure login or not. How do you know if secure login is available? Simple: the URL begins “https” instead of “http” and most browsers will display a padlock (or similar) icon.
Take care too when using Firefox addons which access your online services; many addons don’t use HTTPS.
Sites that I use which offer HTTPS login:
Any Google site; WordPress; Facebook; del.icio.us; Flickr; last.fm; RTM (optional, not by default); Zoho; my Trac/SVN installation
Sites that I use which apparently do not offer HTTPS login:
Nature Network; Nodalpoint; CiteULike; Scintilla; Slideshare; Technorati; any of my Joomla sites
Feel free to provide more examples or corrections in the comments.