open source alternatives to popular websites

Question

Hello,

The other day I came across Jaiku, which is a well structured microblogging system open sourced by Google.

It surprised me I had never heard about this, so I was wondering what other open source alternatives exist for mainstream websites?

I am not asking about forums, eCommerce, wiki's, CMS's, etc, where the market leaders are already well known open source applications (phpBB, osCommerce, Mediawiki, Drupal).

But what about open source alternatives for:

• Social networking (Facebook)?
• Classified advertising (Craigslist)?
• Jobs (CareerBuilder)?
• Dating (eHarmony)?

Or any other field that is defined by a commercial website, as Twitter does for microblogging.

Answer 1

Well, of course there's Identi.ca, also microblogging.

Answer 2

The ones I know of:

Digg-alternatives:

• Kigg - DotNetShoutout uses it
• Pigg

Dating:

• Dolphin

Social site:

• Elgg
• AstroSpaces

Answer 3

This article has some other good alternatives:
http://9lessons.blogspot.com/2009/11/free-clone-scripts.html