Walk For a Dog by Wooftrax is a FREE app. It uses GPS to track you when you are out walking your dog. You are able to select a shelter of your choice to walk for and every mile you walk with your dog earns the shelter of your choice a donation. It is a really simple way to donate to a shelter you love and a great way to encourage you to get out and walk with your dog.
You also do not necessarily have to have a dog as there is no way for the app to know if you are walking your dog or not, it just tracks your distance, so you can still help raise money for a shelter just by walking!
This is an awesome app and it works on both Android and iPhones.
Attribution: Images used in this post are not mine and belong to WoofTrax.