I believe that they now make Halotron instead of Halon. Largely the same, but not as killing to the ozone (or something to that effect). The other benefit is that the Halotron leaves no residue if used, whereas the dry chemical extinguishers leave a dust/chemical residue that can mess up your paint (which hopefully the fire has not).
Sean Roe and Roe Racing sells a couple different Halotron extinguishers, complete with mounting brackets (uses existing front seat bolts on the passenger seat - EASY install). I got mine from Ed at Double Venom, and I think that he passed along the mantle to Sean. A good investment for safety indeed!
Chris
PS. I got mine for track time with the Porsche Club (required), but it also impresses folks just from the looks: A mounted chrome extinguisher makes the Viper look even MORE "all-business"!