Back in January we showed off Lightning Timer, an ultra-basic online app that counted down five minutes. If you want a similar feature set but a bit more flexibility, x.minutes.at could do the trick.
In its most basic form, you can simply enter the site address into your browser in the form 3.minutes.at to set up a 3-minute timer which counts down in the tab title. To simplify the timer creation process, you can drag a bookmarklet to your bookmarks for an instant timer running for a specified length.
You can also append a site address to the end if you want to visit a specific site but restrict how long you spend there (though on more complex sites we found this didn't always work so well). When the timer runs out, a simple "Time's up!" warning appears. You'll still need to discipline yourself to return to your previously scheduled task, but it's a potentially useful way of stopping your day disappearing in procrastination.