Based on years of research on "time-boxing", the Becoming Superhuman timeboxer will help keep you motivated to get your work done faster. The timeboxer is also used widely across teams for keeping time during meetings. It's low-tech, unlike using a phone or chrome extension as a timer, so you won't be tempted to scroll or procrastinate.
Key feature: 5/15/30/60 minute time intervals. The 30 minute hourglass has a halfway line indicating 15 minutes has passed.