I've been using focusmate for months, it's surprisingly effective (I figured since they can't see my screen I'd go on hn/twitter anyways, but I dont). Not only is it good for staying on task, I find the biggest benefit is committing to be in your chair at a certain time.
I'm surprised that it's free. Kind of wish it cost a nominal amount ($10/mo?) so that I knew I wouldn't be matched with a crazy person.