One of the most consistently popular posts on this irregularly updated site of mine is about using timers in lessons. That post includes several timers that can be downloaded and used in the classroom.
Recently though the timer that I have actually been using regularly is from the website online-stopwatch.com.
The online-stopwatch site has a variety of stopwatches, countdown timers and clocks including chess clocks as well as an online calculator. The timers, which are written in flash, can be used within your web browser and they also offer downloadable versions of some of them.
My favourite is their main stopwatch and countdown timer.
Flash is required to see the timer.
Online-stopwatch has become my timer of choice in the classroom because of the simple uncluttered look and the ability to make it full screen. It has what I need without lots of extra features that just get in the way for day to day use.
There is also a new version in the pipeline, which is exactly the same, but looks a bit nicer.