GRAMPS is an Open Source genealogy program using the GTK/GNOME interface. It is an extremely flexible program fitting the needs for both the amateur genealogist and serious genealogical researcher. GRAMPS has the ability to import GEDCOM files exported from many proprietary genealogy programs and can produce a large number of reports in many popular formats.
I have been slowly building my family tree for many years and it is so nice to have a free genealogy app available that is as robust as this one is. Easily input your family info, create graphs, charts and reports as well as add photos, map links and more.
You can find the latest version available in the Ubuntu Software Center.
You can also find GRAMPS online at http://gramps-project.org/