The Donna AppSiri's got an assistant. The Donna app acts as an electronic manager of your day, reminding you about things such as meetings and the weather.
Calendars By Readdle AppCalendars by Readdle syncs with both the built in calendar and Google's calendar app, letting you manage events online and off.
Carrot AppTo-do lists generally don't have a lot of personality. You write something down… you check it off. But the Carrot app makes to-do lists fun.