From Apple…
You can use the Character Palette to insert special characters, such as mathematical symbols, letters with accent marks, or arrows and other “dingbats,” into your documents.
You can also use the Character Palette to enter Japanese, Traditional Chinese, Simplified Chinese, and Korean characters, as well as characters from other languages.
Open System Preferences and click International. Click Input Menu, then click the checkboxes next to Character Palette and “Show Input menu in menu bar.”
If an application is specifically designed to work with Mac OS X, you can choose Edit > Special Characters to open the Character Palette.
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The text above is specific to Mac OS X 10.5 Leopard. Similar info for older versions is available here.