buildcraft
the powder toy is a desktop version of the classic 'falling sand' physics sandbox game, it simulates air pressure and velocity as well as heat!
home of minecraft forge, allowing modders and developers to extend the minecraft experience.
feed the beast is a group of people that specialise in making high quality modpacks and maps for minecraft.
mod-buildcraft.com Top 5 competitors are computercraft.info, feed-the-beast.com, minecraftforge.net, powdertoy.co.uk