If you wonder what kind of pomodoro I'm going to speak about, see the site of the Pomodoro Technique.
When working or studying in shared places, a pomodoro cannot be used. A replacement should in my opinion satisfy the following requirements:
There is a simple replacement which meets all the requirements above: a pomodoro in mp3 or other audio format. You play it on your computer or on a portable audioplayer and listen to it with a headset.
So I recorded two real pomodoros loading, ticking, and ringing - the one for 25 minutes, the other for 5 minutes. You find them below, encoded as variable-bit-rate mp3 and flac audio files (right-click and choose "Save as..." or similar):
PS: The "Pomodoro technique" originally started with some insights, advice, and a booklet shared for free online. Unfortunately it has now become a money-making business. Sigh. Here I share the original, free version of the Pomodoro Technique book by Cirillo.