Sora is Hael Joel Osment (Sixth Sense, Second Hand Lions).
Kairi is Hayden Pattentire (Racing Stripes, Remember the Titans, Raising Helen, Ice Princess, Joe Somebody, Princess Dot in A Bug's Life, and the Disney Channel Movie Tiger Cruise. [She's a totally bodacious babe, if I may so bold as to say so]).
Aerith is Mandy More. (Yes, the singer.)
Leon is the guy in Buffy the Vampire Slayer and Angel.
The ninja Yuffie is Christy Carlson Romano, who plays on Disney Channel (Ren on Even Stevens, and Kim Possible, the movie with Hilary Duff called Cadet Kelly) and is Belle in the musical Beauty and the Beast on Broadway.
Sephiroth is Lance Bass, singer in the one-hit-wonder band N'Sync.
Ansem is Billy Zane (the rich pants lover with the diamond in Titanic).
Phil is Danny DeVito (most recently in Big Fish, but he was the original Phil).
Hercules is Sean Astin (who we all know as Sam on Lord of the Rings and the Goonies, and 50 First Dates.)
Hades also has his original voice, James Woods. Not much to his name other than Hades on the Disney Channel and in the movie. He's also lent his voice to Disney's Recess: School's Out and Stuart Little 2. But he's in John Q.
And yeah. Sorry, I love that kind of trivia.