Ever since MWSF 01 I believe, Apple has strayed from their trademark Apple Garamond font and has gone with the Adobe Myriad font. This font is built in to Adobe Photoshop, but I've found a way to use it in other applications as well.
Navigate to /Library -> Application Support -> Adobe -> Fonts -> Reqrd -> Base (the top-level Library folder, not your user's Library folder). There you will find the fonts entitled Myriad and Mryiad Pro. Simply duplicate them (Command+D) and place the copies in the Fonts folder in your /Library -> Fonts or in your ~/Library -> Fonts folder. Now they will be availble to use in other applications.
[robg adds: These fonts are included with Photoshop Elements as well, though the "Pro" font is named MyriadPro-Regular.otf.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030422092408527