http://about.ning.com/product.php
Ning products including removing their ads, their links, your own domain, and extra bandwidth. The full suite costs $33/month plus $10/month per 10Gb disk / 100Gb bandwidth.
http://brightkite.com/
BrightKite is location based social networking. It will post your current location to twitter, and other cool stuff. Currently invitation only (unlimited invitations).
http://buddypress.org/
BuddyPress is a series of plugins which transform a vanilla WordPress MU installation into a social network. It's due for final release around Dec 2008.
http://communityserver.com/
Community Server is a white label social networking platform. It runs on Microsoft .NET / IIS technology. It powers the MySpace Forums and Microsoft's adCenter. Starts from $3k per year (small site), $50 per seat (min 200 seats) or $20k per server.
http://ping.fm/
Ping.fm allows you to update a bunch of social networks in one go. Post to ping.fm and have it passed on to Twitter, Facebook, plurk, myspace, linkedin, plaxo, bebo, etc, etc.
http://www.forbes.com/style/2008/05/02/social-networks-vip-tech-personal-cx_nr_0502style.html
An article outlining social networks that target the rich. aSmallWorld is apparently the most popular, where only certain members can invite new members.
http://www.kickapps.com/
KickApps is a white label social networking / video hosting platform. Free to sign up, ad supported, or you can buy the ads out, starting at $100/mth for 5'000 page views.
http://www.moggl.com/
Moggl is a social networking site. I met a developer who works for moggl at a Cape Town GeekDinner.
http://www.owjo.com/
Sell on facebook and other places. They handle everything for 7% plus 3% + 30c for card processing, but they'll take cards for you, no merchant account required, through elavon.
http://www.socialengine.net/
Social Engine is a white label PHP based social networking platform. You can implement your own plugins around their architecture. Basic costs $300 plus $290 for all the plugins ($30 or $40 each) as at June 2008.