From social networking to blogging to image sharing…. a nice list of all the basics organized by category.
A few additions:
Under social networking, I would add one of the oldest social networking sites for artists, deviantart; under misc, I would also throw in sites which are more tool based, too, like slideshare (for sharing powerpoint), and some of the rss/blog services (technorati, bloglines, google reader) and maybe most importantly, claimid, for identity management.