Orzeszek Timer is my favourite choice for free computer timers (great of keeping an eye on time if you constantly find your time flying past you). It has a simple interface which allows the input of time and allows many variations of the same time (eg. 25m and 25 minutes or even 25).
The timer can be run with a shortcut or command line to allow it to time the same amount each time the shortcut is clicked. The program is only available for Windows computers and like all great timers includes a ringing alarm which supports custom .wav files. Note that the program requires Microsoft .NET Framework 3.5 for the program to work.
Download at Orzeszek website.