The fonts panel is potentially available to any application, but most companies porting applications from Mac OS 9 do not incorporate it. Pretty much all Cocoa-written applications and most newer applications do use it though. To see it in action, open Text Edit and hit apple-t or choose Show Fonts from the Format menu. Other Apple apps that use it include iPhoto, Mail, Stickies and iChat.
However, there is some solace in that there is a universal character palette that you can turn on in the keyboard system preferences. When you check this option there, a small icon will appear in your menubar and you can bring up this panel from there at any time.