Jasper’s Google Reader Subscribe (User Script)
This Greasemonkey User Script for Firefox adds a little icon to the top right of every page to let you know if a feed has been auto-detected and if you’ve already subscribed to it.
If the page has multiple feed options, it’ll display them on rollover so you can subscribe via Google Reader with a single click to your preferred feed format.
Google Reader Notifier Firefox Extension
Not content to just tell you that you’ve got new unread items, it’ll tell you how many in each label if you hover the cursor over it.