I have used this site for an online count down clock
Its pretty good, and simple to use on your smartboard/teamboard. A teacher at school asked me to find a visual clock that looked better than just counting down numbers. We found this one, which is a nice big pie count down, and looks much better.
Both have a open in full screen mode which is great.
Just a minute ago I realized that underneath the link to the actual timer, amid the ads spread all over the site, there are links to a PILE of other clocks, including a candle, bomb, and some other things.
Just something simple that I thought you might enjoy using on your boards.