I had to use a countdown for a project. As I hadn’t so much time to considerate deep implementations step I was looking for something free, already packaged and easiliy customizable. Finding this in wordpress plugin was not easy. But Finally I got one : T(-) Countdown :

• It’s free
• It’s animated
• It’s easily customizable though CSS
• You can embed it into sidebars, page or posts

If you are looking for something similar, have look here