My buddy Gene told me about a cool tool that is very helpful with time boxing. The tool is called "TimeLeft" and it is a very versatile tool for creating countdown timers, activity timers, and things like that. When you install it, it creates a "Happy New Year" countdown timer by default, and you can create your own timers. You can also customize the appearance of the timers, so you can tell them apart on sight - they just hover over your Windows desktop so you can glance at them to see the status (it's not as distracting as it sounds.
Countdown timers can be quite useful. For example, I created a countdown timer to help me keep track of how many shopping days remained for me to get my Christmas gifts delivered on time. I am currently tracking the time left until I have to turn in PowerPoint slides for a project at work. Yes, I often just set reminders in Outlook, but for some high-priority projects the countdown is just what I need to remind me to focus on the project.
You can create one or more general purpose timers with TimeLeft, and each timer has a pause and a reset icon on it so you can restart the countdown. This is useful for time boxing and GTD (create a 2-minute timer like the one David Allen recommends), so you can keep projects or sub-projects from taking more time than you intend.
Timeleft is very cool -- way better than I thought it would be from Gene's simple description -- but its amazingly simple, yet customizable, interface is powerful. On the site, you can see the plethora of features included in TimeLeft.
Try it, and I think you'll like it (Windows only - sorry Mac folks). There is a free version and a pay version, with the main difference being that the free version limits the number of timers you can use at any given time.