Someone else might chime in with a freebie. I spent less than $40 on Countdown Creator (
http://thecountdowncreator.com/).
You can insert background images. But what I do is render the countdown against black or green and key it in editing software so I can have more fun.
Oh yeah - if you just need a countdown on the screen for seminar speakers, breaks and such, there's always
http://www.online-stopwatch.com/. You can download an executable version from them that runs if you do not have web access on the computer.