This is a great free online tool for making to do lists. It allows you to set up due dates, priorities, repeating tasks, and a bunch of other cool stuff. You can also maintain multiple lists in this program. Some examples of lists that I maintain include project ideas, gift ideas, things I want to read, watch, or listen to, etc... It also has a nice mobile application for several different phones (I use the iphone version) and it has a gadget for my igoogle homepage. Most people use calendars to plan out the nitty gritty details of their day, but I think a good todo list is far more effective.
Key features:
1. Create multiple ongoing lists
2. Set up priorities and due dates for tasks
3. Lot's of ways to input data like igoogle, phone apps, and their website.
Science use case:
This is the best way that I have found to plan out my experiments for the week. It also allows me to set up repetitive tasks like changing cell culture media. If you don't get to an experiment during the day, no problem, a click of a button allows you to postpone until tomorrow....much more efficient at moving tasks around than calenders.
Video tutorial: