Facebook
I find Facebook an easy way to communicate with friends. I not really into creating a bloated profile page, so I try and keep it as simple as possible.
Youtube
Probably the best way to do nothing with your time. Engage in community discussions or watch music videos, it's all good. Feel free to subscribe!
Flickr
What a nice place to publish my photographs. Flickr creates such a cool community around your photos. The only pitty is that the limited, free, account doesn't allow you to create more than 3 sets.
Blogger
Blog hosting, simple publishing etc all free! Blogger is an excellent service. I use it for my weblog.
Twitter
My least used Web 2.0 Profile. It would be an awesome way to bring traffic to your website using Twitter, you got to have a lot of followers though.