What to see a social graph of your friends and who is replying to you? With Twitter Friends you can do just that.
A nice graphic representation of the who is replying and linking. It also gives some interesting rankings about other fun twitter facts.
If you do feel like logging in, you can see who in your network is inactive, and who's online - presumably based on the time of their latest tweet. This can be handy if you're looking to prune back an overgrown network. Mostly, though, Twitterfriends is a fun way to find out about and evaluate how you're using Twitter.