DiggTop is a very cool application that lets you check out Digg articles via the desktop as opposed to sparking up a web browser. While I don't think its revolutionary, I do think its a good example of an Apollo app that *I* would find extremely useful. Digg is a monster to navigate at times and the DiggTop UI just makes it so much nicer to navigate.
Also, if you're into building RIAs and Web 2.0-style web apps, you should really be checking out Mashable. It truly is an up and coming social network news site and I've been finding the content much more timely, focused, and precise than TechCrunch, Read/Write Web or ValleyWag. Also Pete Cashmore, owner of Mashable, is *VERY* active with his users and actually responds to emails. Considering that his inbox must be swamped, I find the latter to be very refreshing and indicative of a person that understands how important his community is. Great job Pete.