I practically run my life with RTM. To-do lists are easily manageable and shareable across computers and my phone as well as with other people. They have a great bookmarklet where I can add tasks from a particular Web site, or I can easily e-mail tasks to myself. I can list locations and URLS and notes for any task, and have them repeat if I choose. Best of all? Their iPhone app buzzes me when a task is due and lets me know what tasks are due nearby.