Apple's Reminders app, introduced in iOS 5, works perfectly well at notifying you according to your time- or location-based criteria. But a new app called Checkmark (99 cents for a limited time) gives you location-based reminders using your GPS and important locations you enter, and with a great-looking interface that's easy to navigate.
You start by adding locations important to you such as Home, Work, the Gym, and a friend's house you often visit. You can locate yourself via GPS, but you also can search for an address on a map and add a location pin that way. The next step is creating reminders based on your locations. I chose a few that pertain to my life, like "Don't forget your iPad!" tied to my home location so I would remember to bring it to work. I also have to do laundry this evening so I put in a reminder to "Do laundry!" when I enter into the radius of my Home location. You also have the option to set Checkmark to remind you a set amount of time after you arrive. This might come in handy, for example, if you wanted to ask about a book your friend owned, but didn't want to mention it the second you walked through the door.
That's the cool thing about Checkmark: not only will it remind you when you've entered one of your locations (or a set time after), you can also set the radius for each location. When creating a location you have the option to set the radius at 100, 150, 300, 800, or 1,600 feet (the app defaults to metric units, but you can switch it under the iPhone's General Settings). This means you'll be able to set reminders as you approach a location, such as, "Don't forget to pick up milk!" as you near the corner store by your house, even though the app is using your Home location for the reminder.
Checkmark offers the more standard time-based reminders as well, letting you switch between When and Where by using a couple of buttons at the top.
While I wouldn't consider Checkmark a necessity (the Reminders app will work perfectly well for most users), it offers a clean interface, good-looking icons you can choose for each location, and a couple of added extras (like location radius) to make it worth checking out. If you're looking for a reminders app that has a bit more than Apple's offering, Checkmark is a good choice.