The best open source alternative to Ancestry is Gramps, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Ancestry and six of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Ancestry are webtrees (Free, Open Source), Ancestris (Free, Open Source), GeneWeb (Free, Open Source) and LifeLines (Free, Open Source).
Webtrees is a free, open source, website based, collaborative genealogy application. It works with standard GEDCOM files and is installed on a web server using PHP and MySQL (or Postgres, SQL Server, or SQLite).
Ancestris is a free software suitable for all genealogists. It is available on all operating systems. It strictly respects the GedCom 5.5 and accepts 5.5.1 standard as it directly writes all information into the Gedcom file.
GeneWeb is a genealogy software program with a Web interface. It can be used off-line or in a Web environment. It uses very efficient techniques of relationship and consanguinity computing, developed in collaboration with Didier Rémy, research Director at INRIA.
LifeLines is a genealogy program to help with your family history research. Its primary strengths are its powerful scripting language and the ability easily import and export information in the GEDCOM format.
Geneotree is an open source web-based genealogy application developed in the PHP scripting language and the MySQL database. It installs on Linux and Windows and works natively with GEDCOM files. It makes a variety of reports as PDF files.