Astrid is among the very oldest apps in the Android Market. It is a high quality and feature rich to do list manager. You will find every thing you need in this task manager application. Recently they also added the most important feature that I look for in these apps, Google Tasks Sync!
In fact it will sync with Google Tasks, Astrid.com, and Producteev.com. Within the Astrid.com application you can create lists of tasks and share/assign tasks to other users within your organization. Its really an amazing productivity revolution for Android. It could mean sharing a simple shopping list with your spouse or it could mean a much more complex system for you to assign tasks to your staff or team.
It also has the most reliable notification system that I have ever seen. There are many different options that allow you to really design notifications catered to you and how you operate.
Astrid also has a locale plugin. The locale plugin is a location based system designed to help you created location based profiles. With the locale app and the Astrid Local plugin you can set notifications for your to do items based on location.
With the Astrid Power-Pack you can also get a series of different size widgets and voice reminders. If you try it you will be hooked!