Astrid is a to-do list management service that functions on the web, on Android devices, and on iOS devices. To get started using Astrid you can register with a Google account, Facebook account, or you can register for an Astrid account.
Once you're registered you can start creating lists of tasks to complete. Astrid
provides four default list categories but you can add more if you like.
Each task that you enter can be assigned a due date. Astrid can be used
collaboratively. If so desire you can share your task lists with others
in Astrid. (From Free Technology for Teachers)