iOS: To-do apps aren't difficult to come by on on the iPhone, but Get Tofu's collaborative elements, push notifications and mission structure makes it a bit more fun to use than the average to-do app.
Instead of a typical to-do list, you assign missions to your friends or family. For instance, you can send a mission to your roommate to grab cat litter on the way home. They'll receive an initial notification, and then another reminder later in the day. It's simple, easy to use and free for the ad-supported app. While it's not much more complicated than sending a series of text messages, the to-do list nature and reminder element are helpful for creating collaborative to-do lists where you actually have an idea of what the other person is doing. The only downside is that both people need to have the app installed for it to work. It's a free download in the iTunes App Store.