This is a trick for any application that uses the fonts palette, such as TextEdit or iWeb (which is most Apple apps that involve text editing, in fact; Adobe apps and Microsoft Office have their own font selection tools).
This trick lets you rapidly cycle through various fonts using text from your document so you can see how they look. Perhaps surprisingly, this isn’t easily achieved normally.
Give it a try by opening TextEdit, then entering some text. Highlight it, then open the fonts palette (Command+T). Now, although you can click to select fonts in the list and see the formatting applied instantly to the document, if you select a font and use the Up/Down cursor key to try and cycle through them, it doesn’t work. The cursor is returned to the main TextEdit window because OS X assumes you want to start typing again.
The solution is to select a font, then click once in the search field at the bottom of the palette window. Then click the font again and you should now find that you can use the Up/Down cursor keys to rapidly move between the fonts and have the formatting applied to the document with each new font you select.
If this sounds a little hard to understand, watch this video. First I show what happens if you try to use the up/down cursor keys to cycle through fonts. Then I use the trick of first clicking in the Search field.