I have just started using NewsFire, a new Mac OS X RSS aggregator. It is a slick looking application, that really uses Mac OS X's Aqua look at feel. The author also has written Acquisition.
I had been using Shrook, but it seems like it doesn't work that well if it is not able to connect to shrook.com. The one feature that I would love to see in NewsFire is the ability to mark articles.