Countdown (free) is one of those "I should thought of that" apps. It's simple in concept, but useful all the time.
Countdown has an attractive UI for adding upcoming events that are important for you to keep track of. A birthday, a vacation, a deadline for a payment... anything really. You enter the events into Countdown and can even pick an appropriate photo of a person or place to make the screen more useful or attractive. After that, you're done. The countdown clock starts timing months, days, minutes, even seconds if you like, until the event.
You can set reminders for each countdown, have the app play a selected tune from your library when an event occurs, or share countdowns on Facebook and/or Twitter. Yes, your iOS calendar and Reminders app keep you on top of events too, but I like that Countdown puts all the events in one place with timers, and you can order those events by date to figure out what's urgent and what's not.
Countdown for iPhone
If you run out of photos in your library, there are some in-app purchases of other themes, but I don't think they are really necessary. It's pretty easy to find something on the web that will work for you with a quick image search, save the image to your camera roll and then use that image.
It would be nice if Countdown automatically scoured your calendar for upcoming events so you don't have to re-enter them, but that's the only downside I can see to the app.
Countdown is a handy and free app, and you might find it as useful as I have. Consolidating all your to-do's into a page with timers is going to be pretty useful for a lot of people.
Countdown requires iOS 7.0 or later. It's optimized for the iPhone 5.