GTD Timer is a Windows application that does one thing very well — it is a countdown timer.
As the GTD Timer website freely admits, these folks are hardly the first to create a countdown timer for Windows, but they have easily created the best countdown timer for Windows.
The software is free to use for personal use, and $19.95 to register (which eliminates a nag screen among other things).
From my experience with trying numerous countdown clocks, this does the best job of actually looking half-way decent in an overlay so I can focus on my work while also keeping the occasional eye on how much time of the allotted time remaining I have to finish a task.