One of the great oversights of the iPhone 1.x software was the lack of a "To Do" program, something that was standard on the Palm OS. I never realized how useful this kind of tool is because, frankly, "To Do" on the Treo sucked. For a mere 1.99 you can have a very decent list tool for all the things you need to get done. It's call My Lists.
I have tons of things to do in my life (don't we all), but getting a handle on it was more then the craptacular "Notes" program was capable. I just went on having an extremely disorganized life, using iPhone's calendar for important things, but my little projects went neglected because the calendar program demands you put in a set time. That's why I like My Lists. You make up list categories like "Everyday", "Work", "Groceries", and then add items accordingly. Once they are done, simply tap the item and a satisfying check mark appears. You can delete the item if it's a one time thing, or just uncheck it is a recurring list, like "Everyday". I used it for the first time today and I was much more productive. The only complaint I have is that it would be nice if there was a master "uncheck" option for the recurring lists. As it is, at the end of every day I have to uncheck each of my "Everyday" items. All in all, not that bad. I highly recommend this program due to it's versatility and ease of use.