As an engineer, one of the things that warms my heart is when users of my product become invested enough in it to want to extend it. I was very happy to see that Reader has begun to attract all sorts of hacks and add-ons that tweak the application to better suit the usage patterns of particular people. It's hard to make an interface that is everything to everyone, and add-ons like these are our best hope of making Reader the ideal feed aggregator for the largest number of people.
Here are a few add-ons that we've discovered recently:
Google Reader Notifier (Mac): This open-source application adds a menu-bar icon which keeps track of new items within your entire reading list or just for a particular tag (the latter is useful for people like me that are subscribed to a lot of feeds and want to know only when the important ones are updated). Troels Bay, the author, has been revving the application on a regular basis, and it's getting better and better.
Gordita: Reader has one-click starring and sharing of items, but you may want to do the same to pages outside of Reader. Gordita lets you create a bookmarklet that allows you to copy Reader items that interest you to del.icio.us, along with all the other items you've bookmarked over the Web.
Google Reader Optimized: This set of user styles maximizes the reading area. When you want to sit down and power through hundreds of items, you may find this compact, stripped-down interface preferable.
Controlling Google Reader with a Cell Phone : This add-on may be a bit out there, but in a nutshell it allows you to control Reader with your Bluetooth cellphone. Perhaps if used in combination with the full-screen style above, you can build your own Reader 10-foot user interface.
Reader Button for the Google Toolbar for Internet Explorer: If you use the Google Toolbar for Internet Explorer, this custom button gives you one-click access to Reader and notifies you of new items via a changing icon.
Updated at 11am with the Toolbar button.