AlertThingy, the first (I believe) AIR application for FriendFeed, is now out. If you're a FriendFeed user just go and install it. It's probably what you've been waiting for: A desktop application that funnels all the things your friends are doing that make it into FriendFeed to your desktop. You can also post comments back to FriendFeed (and the sites FriendFeed then posts to) with the application.
Most of the content I see in my FriendFeed account comes from Twitter, but I don't find the application to be as clear or well-sorted as Twhirl, everyone's favorite Twitter client. There's a reason for this: FriendFeed is a more socially complex service, since it brings together your different networks. But on the positive side, FriendFeed, and by extension Alert Thingy, allows you track Twitter conversations better than straight Twitter clients themselves do. That's because FriendFeed collects "@" replies to Twitter posts under the banner of the thread starter. It's a bit like Quotably (review).
The application is marked version 1.0, but feels more like a 0.1. Sign up is unusual (it requires you get a key from FriendFeed); and the usually-configurable settings such as window transparency, update frequency, and colors can't be changed. But if you're into FriendFeed, this is the application you've been waiting for.