If you desire to change font in your document (in a Cocoa application), but you don't know which font to use, open the fonts panel (Command-T), and then select a piece of text in your document. Now if you want to select your fonts with the arrow keys, you have to click in a text field like Search at the bottom of the font panel window, and then click in the font list. Now if you press the Up or Down arrow keys, you can select the font you like for the selected text (which will change on the fly with your selection).
[robg adds: This seems to be an odd quirk in the fonts panel; although you can select fonts by clicking on them in the Family column, the column itself isn't "active" when you do this. Instead, as noted, you'll have to click in Search, then click in the Family column. You can tell it's active as you'll get your standard highlight color instead of the gray "not active" highlight color. Once it's active, the arrow keys let you quickly scroll between fonts. You can use the same technique to activate the Typeface column as well.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040505104959283