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.
We also snuck in a small feature with this release: when you subscribe to a feed, you'll get an "Add to a folder..." drop-down. This way, you can move that feed to a folder right there and then, without having to go to the settings screen.
Reading list ordering defaults: Your reading list can be sorted in different ways (by date or "automatically", which shows you more relevant items first). Additionally, it can have read items hidden or always visible. Until this week, the default combination was to sort automatically and show read items. Unfortunately, this meant that new items did not necessarily end up at the top, which was confusing to some users. We've therefore changed the defaults to sort by date and hide read items. If you prefer a different combination, these settings can be controlled by the links at the bottom of your reading list (pictured on the right).
Reader has had an inline podcast player for a while. However, one of the core attractions of podcasts is that they can be taken on the go. As a web app, Reader can't really function as a 
