There are many to-do list apps available but finding one that actually integrates with other services and that actually works is hard to find.
The Astrid task manager/ to-do list is a great app that provides task prioritizing, sharing, and push alerts that has been available to Andriod users for quite some time now. The Astrid app is now available for FREE to download from the Apple app store for iOS devices. I downloaded the iPhone version after setting up an account at Astrid.com.
Your account at Astrid.com integrates with your email or social networks to be able to share tasks with others. You can also share tasks with others even if they are using Android devices. There is a cute playful reminder system when you miss a task.