Many of Apple's applications have gone the route of being self-contained packages, hiding their resources from the casual user to protect file structure integrity. This has also allowed Apple to include many interesting resources, such as extra fonts, with these applications.
Extra fonts can appear in any application, especially ones that include templates or customizable production tools. Aperture 3 includes Hopper Script, which can be used in creating books and other print materials. In iPhoto '09 you can use Bradley Hand ITC. iMovie '09 and iDVD '09 seem to be the mother lode, though, including over 15 fonts.
Extra fonts include Coolvetica, Duality, Handwriting Dakota, Strenuous, and more.
Make these extra fonts usable across your Mac
First, choose a program where extra fonts may exist, such as Aperture. In the Aperture application package (Applications > Aperture > (Right-click Show Package Contents) > Contents > Resources > Fonts), the nonsystem font included is called HopperScript. To install, double-click the HopperScript.ttf file.
Font Book will open and give you a preview of the font. Click Install Font and your font will appear in Font Book. Fonts included in Apple-developed applications should be fine. If you find extra fonts included in third party programs, you may want to then validate your newly installed font using Font Book. Simply right-click the font and choose Validate Font from the contextual menu.
What other applications have you found on your Mac with cool fonts? Let us know in the comments!