Diacarta is a to-do app like you've never seen. I saw it on the excellent Well-Placed Pixels blog, and the design of the thing is really interesting. Basically, to add items to the to-do list, you just add icons to the image of the clock. Once items have been added, you can see a visual representation of what your day is like. I'm not so much concerned about keeping a strict calendar as I am about just having a list of my various tasks during the day, so for someone like me, Diacarta is a new and interesting way to look at how my tasks line up.
Unfortunately, it's probably a bit more style than substance for serious calendar users; it doesn't sync up with other calendars at all (so you have to put everything into the app yourself), and it won't send notifications or let you know when something you plan is actually happening. That means it's probably not a satisfactory replacement if you already regularly use some kind of calendar.
However, as a simple visual representation of a day full of errands or various tasks, Diacarta looks great and works just as well. If that's something you're interested in, consider ponying up the US$1.99 price.