iPhone: Countdown (Free) — This plain, but pretty app allows you to customize the event you are gearing up for with a personal picture. Use your sonogram for baby's scheduled day or birth, or a cake for family birthdays.
iPad: Countdown to an Event (Free) — Use this no-nonsense app to count down to an event by days, hours, minutes and seconds.
Droid: Countdown (Free) — Dress up your Android with this widget-inspired app that can show the countdown as your background image.
BlackBerry: Best Countdown (Free) — Create a countdown from your BlackBerry's calendar, and display it in app view or screensaver view.