NotifyMe is an app that I reviewed back in 2009, and almost two years later I still use it all the time. Whenever I need to remember to do something at some point in the future, such as remember to call someone, pick up dry cleaning, etc., I just launch the app, type a quick reminder and enter the time and date. At that time, the app plays a sound and pops up on the screen and tell me what to do. I mention the app today because the main version of the app normally costs $2.99, but it just went on sale for $0.99. The app also has a nicer interface than when I posted a full review back in 2009 and the current 2.0 version has additional, sophisticated features.
The version that is now on sale for a buck syncs with a cloud service, so you can go online to manage your reminders if you want. And there is also a $7.99 iPad app that gives you some of the same control. But I also see that there is a free version that doesn’t sync with the cloud and doesn’t allow you to set up sophisticated repeating reminders, but it does make it easy to provide simple reminders. Frankly, that free version seems to include everything that I do in the paid version, so if $0.99 is too rich for your blood, I encourage you to download the free version and see what you think. Hopefully the app will help you as much as it helps me.
[UPDATE 5/21/2011: Note that while the developer calls the app “NotifyMe” and that’s how I have always referred to it as well, make sure that you don’t confuse this app with another app on the App Store called “Notify Me.” The full name on the App Store of the app that I use is called “NotifyMe 2: ToDo in cloud.” Make sure that you get the correct one, and if you click any of the below links, you will.]
Click here for NotifyMe (on sale for $0.99): 
Click here for NotifyMe Simple (free):
Click here for NotifyMe Control Center for iPad ($7.99):
Jeff, I have been using Futuregram, also $0.99, and I have been very satisfied. It has reminder repeat as well for those “gotta get it done tasks.”
Your information does not seem entirely accurate…when I visit the App Store, “Notify Me” is by one developer (K.I.D.), and has zero ratings, and the other two are by “PoweryBase.” Your link above under “NotifyMe” actually goes to “Notify Me 2: To Do in the Cloud” which is $5.99.
[Jeff responds: Thanks for the comment. I updated my post at the end to make it clear which app I am talking about. You are correct that the sale is now over so NotifyMe once again costs $5.99. It is a really useful app, worth the $6 in my opinion, but you can also try out the free version to see how the app works. As I noted in my post, for many people the free version may well do all that they need.]