Gramps is a Free Software Project for Genealogy, offering a professional cross-platform genealogy program, and a wiki open to all.
It is a community project, created, developed and governed by genealogists. You can install third party plugins to give Gramps extra functions such as filters, reports, tools and many others. When you gather your data you can create, edit and store your family tree.
Also, you can write notes and create events for each person that is in your family. Some examples of events would be birth, marriage, children, divorce, death etc. When you are done inputting all your information it is possible to create data and graph reports in many different formats. More Info »
Gramps has a good Wiki page where you can learn how to use the program and gives you information on how to find the best sources for your genealogy research project.
Downloads are available here: http://gramps-project.org/wiki/index.php?title=Download
A portable version is available here: http://portableapps.com/apps/education/gramps_portable