Have you ever been reading an email in Leopard's Mail.app and thought, "Oh, I need to remember that"? If you can right click, then you can remember anything, quickly and easily, using Todos. When you come across something in an email you want to remember, first highlight the text.
Next, right-click on it and select "New Todo." The current message window will then show a notepad-style view of the selected text. From there, you can change the due date and priority. Finally, assign that todo to the iCal calendar of your choice by clicking the orange arrow/circle beside the item.
Mail displays Todos in the sidebar. To see your tasks in iCal, select "Show To Do List" from the "View" menu.
Want more tips and tricks like this? Visit TUAW's Mac 101 section.