Ever heard of Twitter? Someone invited me to join it today and for some reason I did. I honestly don't know why because I told myself I wouldn't be joining any more networking things. It's kind of good though I have to say. It's like Facebook and MySpcace without all the dangly crap.
The entire network is based around the question "What are you doing?" I think you're supposed to answer the question at least once a day to get the full experience out of the site. Of course, I quickly learned that if no one you know uses Twitter, its usefulness is incredibly diminished (much like any other social network).
What is cool about it though is that you can post what you are doing via SMS on your mobile phone. I did this today to test it out and it works fine, although I don't think I'll be doing it terribly often because it's so expensive (international SMS rates if you do not live in the United States).
Perhaps more useful to the casual user (who doesn't have a swarm of e-friends they need to keep track of) is Flickr. I've actually been signed up to Flickr for a while now, although I don't use it in its social capacity.
At its heart, Flickr is image storage. All I really use it for is to host the occasional image that I might need to link to for some reason. It works perfectly fine for all of that. However, much like YouTube you can get notifications of when other users add new content, and you can leave messages and comments for your 'contacts.'
What is great about Flickr is that almost every picture uploaded there is under a Creative Commons license, meaning you are free to use the images that appear on Flickr without fear of someone getting pissed at you (subject to conditions that are clearly represented below each image). This makes Flickr an image library which is incredibly easy to search and draw from.
Très handy for web developers and bloggers alike. The quality of some of the photos is amazing!