Flock  

I'm still new at the Facebook and Twitter thing but I've been playing around with Flock for the last few weeks.  It's a web browser built on the Firefox engine but has integrated social networking features directly in it.  You can view your facebook friend feeds directly in a side bar, tweet on twitter directly from the browser and it even has a blog publishing engine in it (I'm using it now).  If you are a heavy social network user I highly suggest it.  The only thing keeping me from going fully over to Flock is the number of add-ins I have in Firefox and Firefox 3 rocks.  There is a beta of Flock built on Firefox 3 instead of 2 but it is a little buggy.