If you do a lot of technical or formulaic writing, or translate from one language to another and need to access mathematical, scientific, Greek, or any number of accents used on foreign languages, try this easy reference tool:
Go to your System Preferences and select the "Language & Text" pane. Select the "Input Sources" tab and click on the box marked "Keyboard & Character Viewer." This will pop a small icon up on the upper right menu bar, next to the clock. Now when you click on it a palette will reveal itself showing most available technical and mathematical as well as foreign accents and letters. When you have a document open and need to use a particular symbol, simply pull down the palette and select the symbol you wish to use and then click on "Insert." This works for most word processing apps including Microsoft Word and Pages. Enjoy.