webtrees is the web's leading on-line collaborative genealogy application. It is open source software, and costs $ zero - yes, that means totally FREE!
All you need to use it is a website with PHP and MySQL installed.
It works from standard GEDCOM files, and is therefore compatible with every major desktop application; and it aims to to be efficient and effective by using the right combination of third-party tools, design techniques and open standards.
http://www.webtrees.net/index.php/en/
Thom van der Molen Genealogie