I tested an available one... called TimePunch.
It does have a stopwatch that runs parallel to whatever you're doing, but only tracks your stopwatch input.
That would be enough for me - as I can then track things like non-computer activities (meetings, analog art sessions, etc.) and assign these to jobs/ sub-jobs. The key for me would be the ability to align these to my on-computer tracking for eventual billing and workflow optimization.
So... if I may vote, then: +1