If you're interested in business networking, Ryze is worth checking out. A relative newcomer, it doesn't have the large networks or polished interface of LinkedIn. What it lacks there, though, it makes up for in terms of local presence. Ryze makes it easy to find and meet up with people in the same city, with RSVP-handling for events.
The basic account's pretty limited, though. You can't contact anyone beyond two degrees, which isn't really that bad because you can (and should!) develop links with people along the way. You can't create your own network, so no Orkut-like groups here. There seems to be a limit on how many times people can contact you. And just what does it mean to "see who wants to network with you"?
On the plus side, you can customize your profile page to an insane extent. That's cool. There's also more of a social aspect to it, with people indicating their interests, haves and wants.
Something to watch because of the meetups, but my primary network's still going to be