A really simple but useful tool - classroom timer - can be found here.
You will have to open it on the screen or whiteboard.
A fantastic tool if you want to limit the time for some lesson task, e.g. a short vocabulary check. Tell the students they have got 5 minutes, start the clock and leave it on the screen for the students to see it.
The best thing is you can make the timer count up or down depending on your needs.
See more fabulous classroom tools on http://www.triptico.co.uk/.