For all your stopwatch, countdown and timer needs, try Online Stopwatch. It has a handful of timers and clocks all of which you can run full screen or you can download their always-on-top app.
Even if you don’t think you need it now, trust me and just bookmark it. I use timers like this all the time, for things as simple as remembering when I put things in the in the microwave or oven, or to remind myself that I have to pick someone up at the airport in 45 minutes… easy to forget when you’re at the computer. Most recently, I’ve been using the countdown clock to help discipline myself to do specific tasks for set intervals of time. For example, I for myself to do nothing but clean around the house or read a book for 45 minutes.