Microsoft Word uses the Symbol command from the Insert menu to insert special characters, or you can open the Character Map utility in Windows (found under Start, Programs, and Accessories, or one layer deeper, under System Tools).
On a Mac, use the Key Caps accessory under the Apple menu, select the font you want, and then hold down the Option key to see the various special characters.
Historical perspective: You can enter special characters directly by using the Alt key and the numeric keypad (the Num Lock must be on). To do this, you need to know the Alt-number associated with each character.
For the Table of Codes to Create Special Characters, click on "more info" below!