Google Reader Watcher is a Firefox extension that monitors your feeds from Google Reader and shows alerts when there's something new to read. The extension shows the number of unread posts in the status bar (this number is only an approximation, because Google Reader doesn't count past 100 for an individual feed). If you hover over the icon, you'll see the list of feeds that have unread posts.
Another way to keep up with your Google Reader feeds is to subscribe to this feed locally (for example, using Firefox Live Bookmarks or Opera's feed reader): http://www.google.com/reader/atom/user/-/state/com.google/reading-list?n=100. Note that the n= parameter represents the number of items from the feed and can be adjusted. As expected, you need to log in before accessing the feed, because it's not public.