I will second dirkdj here, although not free, and I don't really see it as a saving of any kind unlike dirkdj.
I personally I believe that my manual day navigation skills are good enough not to waste 4 (or more litres of avgas) per month that I wouldn't waste on following GPS!
BUT it is an incredibly useful tool. I don't use half the features - but the NOTAMS, moving map, frequencies, plogs, recording flights (to later analyse them, or even publish them if you like), and nearest VRP with a distance and direction are incredible features. Especially when you are flying over or in a layer of cloud!! It also has up to date charts which if you decide to fly to mainland Europe you can print as a backup, saving on purchasing numerous maps (although personally again I also purchase those).
There are thousands of apps out there - I have tried a lot of free ones, and although good - not fantastic and truly for that price unbeatable.
either way I would recommend purchasing a separate GPS dongle which connects via Bluetooth. As this dramatically improves reliability, and ensures your position is current.
Good luck in your search!