Astrid is an amazing task management application for the Android. It handles virtually everything task-related task you can think of. It lets you set priorities for your tasks, deadlines, and even keeps track of how long you spend working on the task, which makes billing much easier.
For those of you who use it, it can sync with Remember the Milk, which makes it much more convenient to keep track of things on your computers as well. It also has a widget for the Android background that works pretty well too.
Astrid is free, and you can find more information about it here.