Astrid is a combination of to-do list and social media. It is similar to Remember the Milk in many ways. You can create to-do lists and list categories as well as due dates. Tasks can also have priorities.
Okay, so all of that is like many other to-do list applications. The added feature is the social media aspect. Like most social media sites, you follow other users which gives you the capability to share lists and assign tasks. For example, your teaching team can all get accounts and follow each other. Then you can share tasks. This works great when planning grade level or departmental projects or events. Need to plan the school carnival or rewrite curriculum? Get everyone on Astrid and share the work.
Astrid currently has an excellent Android app. I’m not sure when an iPhone app is due.