MadCap is the big one I hear about, then maybe FrameMaker at a distant second and plain old Microsoft Word. As a software technical writer who loves docs-as-code I don't know how the writers in manufacturing and aviation and whatnot put up with those tools...
MadCap Flare was an awesome product. It grew out of the old BlueSky "RoboHelp" suite (which was also awesome) and could be used to output to many different formats (online help, web help, print docs, confluence). It's been a while, but I think you could even use FrameMaker to author and still use Flare to transform output to all its supported formats.
It's been a long time since I immersed myself in this world, so apologies if my memory is less than perfect.
They have local chapters in many geographies. Their annual conference is usually well attended and covers topics related to technical writing from Help Authoring to Content Strategy to Content Marketing to Agile.
