That's a great service. I can think of various features to support more use cases:

* provide an optional label - for the use case where several clocks are used and shared.

* start and share several stop watches from a single page: start them one by one or together. For a race with multiple participants that either start sequentially or together.

* Support taking split times.

* Combine timer and stopwatch: count down from a set time, then start the stopwatch at zero. It currently does this already. This supports races like sailing where the actual start is preceded by a countdown period.