During most of my workshops I am using hourglasses. It works great for facilitated meetings and standups.
Next to that it works great to have them in a meeting room.
The rules for a meeting are: anyone that thinks that a discussion in the meeting is taking too long, can turn it around and tell his co-meeter: lets discuss this only for this amount of time. After that the people vote together if they want to keep on discussing or go on.
Last Monday during another successful GTD/personal kanban course, I got again the question about where I bought them.