I wrote up a guest post for Richard McManus on Read/Write Web which talked about feed readers and gave a run down on the various types of readers out there. Richard took it a step further and pulled out some of his favorites here on ZDNet (and I agree with his picks). I also want to comment here on the role of RSS in Rich Internet Applications.
I think RSS is so important for the RIA vision because it breaks down the data into simple, easy to consume bits - perfect for a developer creating an application that puts the content front and center. As RSS expands, and we're seeing that with microformats, the lure of pairing the technology with RIAs is only going to get stronger.
This is the reason I think RSS readers are so important. They represent a great first step, and also a great test case for RIAs. One of my favorite applications, Goowy (which I did a review of for Richard here) has integrated RSS nicely into their desktop application. Most, if not all, of the Ajax desktop applications include RSS functionality. While all of these are beneficial, there hasn't been an exclusive RSS reader built as an RIA and that's too bad.
The combination of RSS and experience is very compelling, and new developers can let their creativity run wild with an RSS reader. At the surface, it's very simple. But by building an "Apple-style-reader": one that is elegant, fun to use, and extremely functional, it's easy to get a sense of how different the web applications of the future are going to be from those that we have now. The fact that RSS is now extending into media and audio makes the challenge even more fun.