iOS: TaskAware is a clever app for GPS-enabled iOS devices that lets you know when you're near a location assigned to one of your tasks. For example, if you need to remember to pick up milk, TaskAware can alert you when you're near a grocery store.
TaskAware works like pretty much any task manager, in that you add tasks, set priorities and assign a deadline (if relevant). What it also lets you do is assign a location, so when you are near that location Task Aware will alert you so you can stop and take care of it. TaskAware integrates with your address book so you can add locations you've already entered to save some time. If one of the biggest barriers to you actually getting many of your tasks done is to remember to do them while you're mobile, TaskAware can be a big help.
TaskAware comes in two flavours: the ad-supported version is free and comes with all the features of the paid version, which is $6.