I love Twitter! I learn so much from those people on there! Especially John Pederson. Today he twitted about a great online tool. Here's an online stopwatch for your next class/professional development session. You can put in the time frame you're looking for, either have it count up or count down, and go for it. You can download it for Mac or PC (haven't tried it that yet, but did try the count down for a time out for Joey and his rocket that ended up on my roof). They even have a LARGE version as well.
I once had some videos that did a 2 minute and a 5 minute count down, but lost them some computer changes ago. I like this tool so much better.