Far and away, the best app out there is called Things. There's an iPod/iPhone version and a Mac version, which sync with each other. I guarantee you it's the best to-do app you've ever tried in your life, or I'll tip you $1M. And I'm not even affiliated with the program or its creators.

When I bought it, it cost $10 but it's worth every penny. The desktop version is more expensive, but when you combine the two, it makes for a rock-solid, easy to use system that helps you focus on what you want to do today, lets you put things off for "someday", schedules to-dos, tags them with tags just like mahalo uses, and even categorizes them.

For example, you might have a to-do that:

1. Is scheduled for July 4, 2009
2. Is categorized as Family
3. Has a couple of tags: Errand, and High Priority
4. Is on the bottom of the Today list, but you want to move it to the top... and so you do, by clicking on Edit, and dragging that item to the top of the list.

I used to use Appigo Todo and Action List on the iPod Touch but recently chanced upon this application that i review called 2Do. Thought nothing much of it but once i use it, it turn out pretty great for getting things done. Here is a guide showing how i set up 2Do as my GTD App >> Guide to great iPhone and iPod touch todo list with 2Do

You might want to follow the getting things done methodology in carrying out task execution

Free To Do apps: To Do's by AustinBull Software, To-Do's by RubyRazzi, DoBot ToDos by Aria Haghighi. None of those have lower than a 3 star rating.

I have a program called iHomework that cost $0.99 and it's awesome. It shows you a number on your home screen of how many assignments you have left and you can also categorize them by priority (low, medium, high). Thumbs up for that one!

9-Toolbox is free and has a "Days Until" app in it itself. It's pretty handy. It has 9 apps in one. It also has the "Date Calc" which calculates the days between dates if you have a certain amount of time to do something.