If you're the sort of person who thrives on deadlines, you should take a look at Forcedo, an online task manager that includes a timer to help keep you on schedule.
Forcedo is free, but you do need to create an account to use the site. Adding a task is easy -- just type a name and estimate how long, in minutes, it'll take you to complete. Then click the Start button and Forcedo will count down and let you know when you're supposed to be done, with or without a dazzling array of sound effects.
Forcedo is a very simple app. I was hoping to set up an I Love Lucy-ish cacophony of overlapping tasks and alarms, but the site can only count down one task at a time. Also, duration is measured in minutes, so Forcedo is better suited to jobs you want to wrap up before lunch rather than, say, completed over a few weeks in May.
There are occasions when I'd use Forcedo -- especially when I need to stay really focused on getting something important done in a hurry. I can also see some potential for using Forcedo's integration with Twitter to tweet my status on tasks that co-workers might also need to track. But as a general purpose to-do tracker, you might prefer Doris or Remember the Milk.