Jeet kune do is not a style per se, more of a philosophy, true, but there is definitely a physical foundation of theories and tools which Bruce Lee drew upon to create his art. These were:
Boxing, the
trapping aspects of
Wing Chun, and the footwork, theory, and strategy of
western fencing. He also utilized kicking technique from the french art of
savate (that kick he does in Return/Way of the dragon where the guy with the kicking shield goes flying into the cardboard boxes, that's a
savate kick.) and took
grappling techniques from
judo and
wrestling. (NOT
brazilian jiu jitsu, which is where all the modern jkd people get their ground game.)
However it is also worth noting that Bruce shut down his schools because he felt the individual must learn for him/her self; having 'truth' sold to you is a false truth. It is more valuable when you discover it for yourself.
Basically he felt martial artists, the true JKD men, should seek the truth on their own through experimentation and controlled sparring. If someone else is teaching you a system, even if it's called jeet kune do, you will only be able to express that system. It's like the difference between playing perfectly someone else's symphony, or playing free-form jazz. Jeet kune do is the jazz of martial arts.