The iPhone and Touch task manager application OmniFocus can be used either by itself, or in conjunction with the Mac program of the same name.

OmniFocus is an advanced task manager to help you keep track of what you need to do.
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OmniFocus is based on the Getting Things Done productivity philosophy espoused by David Allen. To understand OmniFocus, you need to understand GTD. Here are the principles of GTD in a nutshell:

Break down your projects and goals into individual, physical tasks, known as "next actions." For example, "Lose weight" is not a next action, but "Google to find local Weight Watchers meetings" is a next action.

Each action should be associated to a project. For example, "Plan the 2009 marketing campaign" is a project. "E-mail sales and marketing staff to set up a meeting" might be the first action in that project.

Each action should also be associated with a "context," which is either a physical location, or a place where you have access to the tools and people you need to do the project. Examples of contexts: At home, at the supermarket, near a phone, at a computer with Internet access, in a meeting with my boss.

Write down everything you need to do as soon as you think of it, then consult and organize your list of to-dos regularly, so you won't worry about forgetting things.

The iPhone or Touch version of OmniFocus and the Mac version let you add actions, assign contexts and projects, check off actions as done, assign start dates and completion dates, and view your lists of actions a variety of ways, including by project, by context, and viewing only flagged tasks.

The version for the iPhone or Touch adds a couple of features not available on the desktop: You can set locations using the iPhone or Touch's built-in location services. For example, you might create a context at your neighborhood supermarket and tasks with that context will float to the top when you're at the supermarket.

You can also create a context using a search on the iPhone and Touch's built-in Google Maps application: For example, you might create a context for a map search on supermarkets, and tasks with that context will float to the top whenever you're at any supermarket.

You can also attach photos and voice recordings to tasks.

I don't recommend OmniFocus to everyone. It's a bit complicated. I like to say that it's good for nerdy people like me, who are by nature so incredibly disorganized they need a rigid, complicated system like OmniFocus just to keep from melting down into a whimpering pile of gelatin. I'm not sure how much I'm kidding when I say that.

The OmniFocus version for the iPhone and Touch can be a bit slow, especially when synching. That's particularly annoying when you just want to add a task quickly, and assign a project and context later. The Omni Group really needs to create a separate application or mode just for rapidly adding tasks.

Still, despite the problems, OmniFocus for the iPhone/Touch is a valuable tool to help disorganized people get a little less scattered.

OmniFocus for the iPhone or Touch is $19.99.

Download OmniFocus from the App Store.