I've previously demonstrated how to use LaTeXiT to create and display complex math formulas and other symbols, and thank you readers for the additional recommendations!
While providing a means to display almost every imaginable existing symbol or character, a LaTeX-based solution may prove too complex and unnecessary for some. For the less "mathletic," Mac OS X's built-in character viewer is one of the easier ways to display characters of all sorts.
To enable the character viewer, open Mac OS X's System Preferences and select the "Language & Text" icon. Once there, click on the "Input Sources" tab and place a checkmark next to the "Keyboard & Character Viewer" box and "Show input menu in menu bar" box. The Keyboard & Character Viewer is now accessible on your menu bar.
Now, displaying special characters on your document, email, or whatever it is you're working on is just a drag and drop away.