Remember the Milk is a free task-management web application. When you start it up, it presents you with a series of lists on which you can post tasks. You can organize your tasks with priority levels, due dates, and tags or throw 'em in a pile and use the search function to find what you're looking for. The entire service can be operated through the keyboard, which makes adding and managing tasks incredibly quick.
If you would like to have your tasks on all your calendars, you can thanks to RTM's stellar integration with Google Calendar, iGoogle, your Blackberry or Windows Mobile phone, Twitter, or even through a bookmarklet (you remember those, right?)
Probably the most surprising thing about RTM is its general applicability. I didn't think I had much use for a task manager, but now I use it constantly to keep track of my school assignments, my work projects, and whatever ideas I want to save for later. Soon, I plan to write a quick-and-dirty widget to display my list of ideas on this blog.
If you want to organize your life, or you just want to keep all your notes in one place, Remember the Milk is the web application for you. Find it at RememberTheMilk.com.