From my answer here:
You need a font management tool of some kind to help you organize your fonts into sets of your liking. There are a number of them out there; Extensis Suitcase Fusion and FontXplorer Pro come to mind.
From the FontXplorer website...
Is font management for you? Font
management is for anyone who uses more
than just a handful of fonts.
Ask yourself these questions:
Do you need to free up your system?
Every font that has been activated on
your system requires the deployment of
system resources to applications. This
can be an unproductive use of
resources, particularly for fonts that
are used infrequently. Font management
allows you to activate fonts solely
for the period of time you actually
need them, freeing up precious
resources so your system can run more
Do you have trouble maintaining an
overview of your fonts? Every
professional creative application you
install adds new fonts to your system
that you may or may not want. Font
management helps you find, preview,
and sort fonts and deactivate the ones
you don’t need.