hxxp://www.best-php-scripts.com/social_networking.htm
http://www.elgg.org
http://www.boonex.com
social engine is also very good
and here is the are some good lists of social software i have found so far:
http://emersondirect.wordpress.com/2008/08/01/25-enterprise-social-media-platformscompanies-to-be-aware-of/
http://www.clone-script.com
http://www.techcrunch.com/2007/08/14/34-more-ways-to-build-your-own-social-network/
http://www.techcrunch.com/2007/07/24/9-ways-to-build-your-own-social-network/
http://mashable.com/2006/07/04/goingon-network-build-your-own-social-network/
http://www.web-strategist.com/blog/2007/02/12/list-of-white-label-social-networking-platforms/
I think ablesp*ace is a very good one. You can get free somewhere here.