The Provari, The Smoketech Vmax, the Darwin and the Reo Woodvil VV, not necessarily in that order and good luck getting your hands on the latter two.
The Smoketech Vmax is a fairly new device and is, in several ways, superior to the Provari, and it's less expensive.
The Darwin is the only Wattage regulated PV, and IMO, that's better than voltage regulation. Good luck getting one though.
The Reo is a hand made one-at-a-time piece of art, limited edition supposedly to be released in a few days to a waiting crowd and, as such, is probably even more elusive than the Darwin.
The fanboys will be along momentarily to expound on the virtues of the Provari.
Then, there is the Russian made PV that I can't pronounce that looks like it weighs 9 pounds, is both variable voltage AND variable wattage, has rebuildable atomizers, fits a 510, is built like a tank, washes your dog and diapers your baby.
You can see pictures of it, and a link, on the variable voltage forum under the thread titled "nivel chip".