It really depends on what you want to use your gps for. Ultimately, I've never found any really good free gps programs out there.
If im off road, either hiking, biking, or geocaching, my favorite program is
Tracky.
On the otherhand, if you are looking for vehicle/road navigation, I have been impressed with
Google Navigator. A lot of people also like Garmin XT, but I have never found it to be very stable on my mogul, and of course the maps get expensive, where as google navigor provides you with the constantly updated google map database for free after you purchase the program.