In our company we're using time estimation as a base for calculating the salary of part-time workers. The problem is that workers don't want to spend their time on it estimation. From the other side, manager want to know which task the worker is currently working on. So it would be useful if worker able to mark one of his task as current and then see how long he working on it. I understand there are sideways such as if worker forget to unmark current task, but this is other question, which maybe solved with help of good design, javascript, redmine or even browser plugins, company politics and so on. The main thing is that this feature will be useful as itself, because it will help to inform worker (and others) about how long he's working and which task is current.