Remember the Milk
Remember the milk (or Rtm for short) started out as kick ass web service for getting-things-done-fanatics and that web service is now accessible on all kinds of devices. Android is one of those platforms.
Apart from being a great list-management tool, rtm for android has few unique features that's not available on other platforms. For starters it can remind you of tasks based on your location. Say that you're passing by the hardware-store and you have some tasks associated to that hardware-store, Remember the milk will remind you when you're getting close to it. Works like a charm.
Unfortunately all this goodness comes at a price. While the android-app itself is free, it requires a premium account at rtm to be able to use it. (You get a 10 day trial period when you install the application) I still love it though and can't really picture my phone without it.
For more Android app recommendations visit my set: Android Applications.