My only connection to this website just got severed. I used to keep
Taco, you killed slashdot. Idle may have been the weapon, but it was you who wielded it.
There are only 4 main engines. Gecko and Webkit, which you already mentioned, then there's opera's and IE's engines, neither of which I can remember the names of at this moment.
There are many other minor engines as well, I call them minor because they are not even remotely html spec complete.