The fonts in this folders shouldn't be touched;
System>Library>Fonts
These are what I have;
Apple LiGothic Medium.dfont
AppleGothic.dfont
AquaKanaBold.otf
AquaKanaRegular.otf
Courier.dfont
Eurostile
Geeza Pro Bold.ttf
Geeza Pro.ttf
Geneva.dfont
Hei.dfont
Helvetica.dfont
Keyboard.dfont
LastResort.dfont
LucidaGrande.dfont
Monaco.dfont
Osaka.dfont
OsakaMono.dfont
Symbol.dfont
Times.dfont
ZapfDingbats.dfont
ヒラギノ丸ゴ Pro W4.otf
ヒラギノ明朝 Pro W3.otf
ヒラギノ明朝 Pro W6.otf
ヒラギノ角ゴ Pro W3.otf
ヒラギノ角ゴ Pro W6.otf
ヒラギノ角ゴ Std W8.otf
华文细黑.ttf
华文黑体.ttf
I added Eurostyle to this folder.
Library>Fonts;
AmericanTypewriter.dfont
Apple Chancery.dfont
Apple Symbols.ttf
Arial
Arial Black
Arial Narrow
Arial Rounded Bold
Baskerville.dfont
BigCaslon.dfont
Brush Script
Chalkboard.ttf
Cochin.dfont
Comic Sans MS
Copperplate.dfont
Courier New
Didot.dfont
Eurostile
Futura.dfont
Georgia
GillSans.dfont
HelveticaNeue.dfont
Herculanum.dfont
Hoefler Text.dfont
MarkerFelt.dfont
Optima.dfont
Papyrus.dfont
Skia.dfont
Times New Roman
Trebuchet MS
Verdana
Webdings
Zapfino.dfont
Your - user>Library>Fonts folder is where you would add fonts for the user.
Apple's page explaining;
http://docs.info.apple.com/article.html?artnum=106417