I love the show, and I consider myself a super fanboy of all things stargate (I own ALL the dvd's of everything Stargate), and was very sad to see it end, but a petition isn't going to do anything, its been on the air for 5 years, which is actually longer than most shows stay on the air, even super popular ones, and for the entire Stargate genre to last a whopping 15 years is unreal and we are very lucky to have it last that long, sci-fi stuff just generally doesn't stay on the air long. There will be a few direct to DVD movies though, as Dorian pointed out, lets hope they don't .... those up like they did to the Stargate SG-1 ones, god those movies were just terrible.
Lets take a look at how long good Sci-Fi lasts to keep it in perspective:
Stargate SG-1 - 10 seasons + 2 movies, a 3rd is in talks
Stargate Atlantis - 5 seasons + at least 1 movie
Star Trek TOS - 3 seasons
Star Trek TNG - 7 seasons
Star Trek Voyager - 7 seasons
Star Trek DS-9 - 7 seasons
Star Trek: The Animated Series - 1 season
Star Trek Enterprise - 4 seasons
Sliders - 5 seasons
X-files - 9 seasons (although a true x-files fan will say 7, when Mulder left, lol)
Firefly - 1 season + 1 awesome theatrical release
Battlestar Galactica - 4 + 2 tv movies
Babylon 5 - 5 seasons + 6 made for tv films.
Farscape - 4 seasons + 1 2 episode miniseries
I honestly would love to see a Stargate spinoff that deals with the ancients and their wars with the Wraith and replicators 10,000+ years ago, as well as how the 4 great races of the galaxy dealt with these things (The Ancients, the Nox, The Furlings, and the Asguard, where as Humans became the 5th race later on), not to mention some of the really cool ancients we met like Merlin or Janus.
Stargate Universe just sounds like a horrible spinoff.