This is Vivalogo's list of best free, downloadable, open source social networking software (kinda hard to say all these words :) ).
Unlike some other lists you may find on the net, this one contains only really downloadable and functional software.
Note: listed in no particular order.
spree

iSocial

Mahara

Yogurt
This is a Social Network module for xoops CMS. You have seen Facebook, Orkut, Myspace , try Yogurt for Xoops!VMukti

The PeopleAggregator

Appleseed
Appleseed is (augmented) social networking software, ie Friendster, only distributed. Sites running Appleseed will interoperate, and form the 'Appleseed Social Network.' Development is focused on privacy and security, as well as ease of configuration.Mugshot Project

GetBoo

Akarru
Akarru is a social bookmarking engine, is used to build social bookmarkings sites, like www.blogmemes.com. Users posts links and promote links to front page using voting system.Scuttle

SemanticScuttle
SemanticScuttle is a social bookmarking tool (based on Scuttle) experimenting new features as hierarchical tags, collaborative descriptions or OpenID authentification.AROUNDMe

Clonesumating

BeWelcome Rox

ICEcore

Memephage
Memephage is an automated web log (blog). It passively gathers and summarizes links from various places. Currently: IRC, social MUDs, e-mail, and web browsers. Uses the POE multitasking and networking framework for Perl.InteractOLE
A platform for the delivery and support of online learning. It differs from many other elearning platforms in that its aim is to concentrate on the social/interactive aspects of teaching and learning rather than the delivery of content to students.Elgg

CommunityNews
CommunityNews uses social bookmarking and bayesian techinques to provide periodic postings to blogs. Users can vote for or against RSS sources ti increase the chances that the source is used again. Spam filtering (bayesian) is provided by spam bayes.OpenPNE
OpenPNE is a Social Networking Service Engine written in PHP. It has many features(friend control,friend invitation,diary,blog feeds,message box,etc).MonkeyChow

NewsCloud
NewsCloud is an open source media platform for citizen journalism and the social news network hosted at NewsCloud.com.Feed Me Links
Feed Me Links stores your bookmarks online so you can get to them anywhere. Import your favorites and share your links with friends. Add tags to organize your links. Discover new things.WorldSpace
WorldSpace is a user-extensible shared virtual environment, aimed at being a next-generation social networking system.Social Networking POC

Zoints
Zoints is intimately aware that online communities are the most important aspect of the internet. Our free software solutions are designed to help solve the three major problems forum owners face: Member acquisition, Member retention and Profitability.earth-life-simulation
A simulation of the world, there will be a global map representation and players can choose a country or civilization and develop it's social, political and military existence.PHPizabi
PHPizabi is one of the most powerful social networking platforms on the planet. With literally thousands of websites powered by PHPizabi including everything from simple friends sites to the most complex networking super sites out there.Ozcode
Ozcode is the source code behind Ozmozr.com, a microformat-aware RSS aggregator, social networking, resource sharing, identity aggregation and presentation site.TallStreet

Jamss
Jamss is a social news site based off of Digg.com. Jamss allows for peer submission and review of web articles and can be adapted to fit a variety of themes. Jamss runs on PHP/MySQL.Dolphin
Dolphin is open-source and free, although there is a cost associated with removing the company�s links from your site.Ospo

AstroSPACES
AstroSPACES is the world's first open source social networking solution. Coded from scratch, it is highly efficient and very easy to use.FlightFeather
FlightFeather's goal is "social networking for everyone". This means that anyone should have a chance to run a popular social networking site -- on minimal hardware, and without wasting bandwidth.SNOSS

OpenSocialNetwork
OpenSocialNetwork is a social network of open source. Its main task is to create a social network releasing the source code.S3B
S3B - Social Semantic Search and Browsing - is a middleware that delivers a set of search and browsing components that can be used in J2EE web applications to deliver user-oriented features based on semantic descriptions and social networking.Facelift
