I've been researching my family tree for over 10 years now and I've tried very many genealogy software programs. Choosing an open source genealogy program was extremely important to me since all the research I've done could be lost if it were locked up in a proprietary file format that is inaccessible 20 years from now. However, it is equally important that my genealogy database be easily accessible by as many family members as possible - family members who may not be very computer literate.
GRAMPS is an outstanding piece of software. When I first began using it a few years ago, it was a little rough and it was difficult to install on Windows. This made it great for me while running Ubuntu, but difficult for me to recommend to family members. It seems to have much better support now under Linux, Windows and Mac. It also uses an open XML-based file format that at least one working group is looking at as the basis for an updated standard to replace the aging GEDCOM format. I think this is definitely the candidate for best OSS Genealogy.
PHPGedView is another good, open-source, web-based genealogy package. This is a good one to use if you're planning to build your family tree collaboratively among several family members. My biggest complaint about it is that it's a little clunky looking and some family members seem a bit intimidated by it, so they don't make changes or additions when they could. I began building a new theme and layout for it, but I put it on hold when I felt like it was taking time from working on the family history itself. Of course, it also requires that you have a server to host it on.
A similar online family tree is Wikitree.com. It looks promising, but I haven't yet found a good way to sync changes between it and my local genealogy software. It's still in its infancy though.
All that said, my favorite genealogy software is the closed-source Legacy Family Tree. The standard edition is free and the full "deluxe" version is inexpensive. Unfortunately, it's Windows only (I've had mixed luck running it through WINE). It's advantage though is that it's very easy to install and use and has some powerful tools for sourcing and merging trees. It also creates some very impressive, customizable family tree charts that can be saved in a variety of formats or printed through their mail-order service. It also saves your genealogy database in several formats including GEDCOM.
Ancestry.com is the necessary evil of the genealogy world. They have many records on their site that aren't available elsewhere on line, but they have made quite a few business decisions over the years that don't sit well with many family historians. They also produce the "Family Tree Maker" software which I recommend avoiding.