Thanks to Stuart Henshall I have just discovered Flock. Flock is a "social browser", due to its ability to interact with social networking sites such as Flickr, Delicious, FaceBook and other services.
It is based on Firefox code and so if you are already using Firefox its an easy move to make. I have been running it in preference to Firefox for the last few days. Its seems fast and stable and easy to use and though I have yet to figure out all the social features - they are AMAZING and I am increasingly loving using it. Looks likely that I will stick with it an ditch Firefox! I feel this could be the way that web browsers will evolve.
You will find a nice little introduction to the product in this recent PC World article by Harry McCracken.
If you are into social networking and use all the common services and products such as Flickr, Delicious andFaceBook then go take a look at Flock - I think you will love it.