Fido Factor - Dog Friendly Guide Fido Factor lists user generated dog friendly spots with reviews and photos. The list is always growing and it is a great way to find new places as well as the dog related rules.
Off Leash This app only lists off leash dog parks but it has pretty good coverage and an easy to use interface.
Pet Phone Great for keeping track of your dogs vet appointments and meds especially when they are a puppy.
MiPets - Pet Info Organizer This app lets you take pics of your doggie friends and add their names. Now you'll never forget the names of the other owners at the dog park.
Dog Friendly UK For dog owners in the UK the website Dog Friendly UK recently launched their app in partnership with the Kennel Club's Open for Dogs initiative. It currently lists over 10,000 locations.
iCam - Webcam Video Streaming The best way to keep track of your dog when you're not home. It is super easy to set up and has a ton of features. I use it with the webcam on my iMac.