Parks Seeker is a free iPhone app developed by Mark Sandell that allows users to track their progress as they visit the national parks of the United States. With detailed information about each park, users can easily check in with their GPS location, share their progress and photos with friends, and create a wish list of parks they'd like to visit next.

The app includes all 63 of the United States' active National Parks, including popular destinations like Yosemite, Yellowstone, and the Grand Canyon. Additionally, Parks Seeker allows users to explore other types of parks, such as National Battlefields, Forests, Grasslands, Historic Sites, and more.

Key features of Parks Seeker include the ability to view an interactive map displaying parks and check-ins, add photos to each check-in, sync check-ins between multiple devices with Dropbox, and share visits through social media platforms like Facebook and Twitter. The app also utilizes GPS location to help users locate the closest park to them.

Whether you're a seasoned national park explorer or just starting your journey, Parks Seeker is a handy tool to track your park adventures and plan your future trips. Plus, the app doesn't require data or cellular service to use most features, making it a convenient companion for outdoor enthusiasts.