(Personally tested and recommended by the author of this web. Some links might lead to other websites.)
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Powerful software system that you have full trust in, and that you can utilize fully and in accordance with your personality, can have tremendous impact on your productivity. No software can solve procrastination issues nor deeply rooted time management problems for you, but again, it can help tremendously. The important thing is, to have all your tasks to do in just one system. More systems make for more confusion and less things getting done.
Above & Beyond is a powerful time management app, though a bit expensive. It has absolutely no competition in it's field - real time and dynamic planning. I like it so much that I am placing here a full article on how to implement the Getting things done approach to organizing your life with Above & Beyond. (Getting things done is a book and methodology by David Allen.)
TimeTo is very similar to A&B, in fact it was built on source code of one of the previous A&B releases. User interface was improved among others, and in general, TimeTo is enjoying continuous updates, unlike A&B which has not been substantially updated for some time already (but new releases were planned).
Outliners are very powerful way of organizing to do lists. If you have a Palm Pilot, Shadow Plan and Bonsai are the best of the breed, Progect is easier and free. All three have a desktop counterpart - so that you can sync your Palm with your desktop PC.
MasterList from Safari Software has a refreshing approach to the traditional todo-list.
Achieve Planner from Efexis is very powerful and has dedicated author. It is a bit complex to master, but you don't have to use all it's powers - after all, how much of the Excel's or Word's functionality do most of the people use, like 10%? You can use Achieve Planner even in a simple way, and maybe as you learn to utilize more of its powers along the road, you will be glad you did. It is a kind of software that grows together with you.
Don't forget the MyLife Organized (MLO) review on this site. MyLife Organized is the to-do list gorilla, outliner on steroids, was great in first release already, and subsequent improvements have been just about fantastic. If you hate outliners, don't touch this one, otherwise you might be nicely surprised. Free version also available. I have used all of the above, and find them all to be very good, yet at the moment, MyLife Organized is the winner for me. Though one of the biggest reasons, I had to decide finally, it would be stupid to use three apps together.
The decision is up to you, no one can make it for you, it's like the way you choose your friends. Some you might like, some not. Don't let anyone to choose for you, test at least few personally before you will settle on one.