There are several tools being developed to get model files into the glTF format, with various feature support. We have built and recommend the Unity to glTF exporter for the majority of models, as it allows you to set the materials using the Unity Standard Shader. There are several other tools depending on your workflow and skills which are listed here, but below is a summary of them.
Blender - Kronos' official Blender exporter: link
Blender is a free 3D model editing tool that comes with gltf import & export options. Actively being maintained and backed up by Kronos, the group that controls the gltf specification.
Unity - Plattar's Unity to glTF exporter: Link
This will require the user to install a copy of Unity and add the tool as a plug-in to your project.
Unity - Kronos's official Unity to glTF exporter: link
This will require the user to install a copy of Unity and add the tool as a plug-in to your project. It is still under development so may be a little unstable but will allow users to import existing glTF models in order to edit and re-export them. At the time of writing this, it does not yet support proper animation exporting but is a high priority.
This plugin is built for Maya 2018 on Windows, with a MacOS version coming out soon.
3DsMax - Babylon's 3DsMax to glTF exporter: link
This plug-in is being actively maintained and improved. It has a very in depth guide on how to use it to export models with PBR materials here.
SketchUp - Centaur's plugin for SketchUp: link
Plug-in to export glTF/glb files from SketchUp
Cinema4D - Exporter in Maxon Labs: link
Plug-in to export glTF files for Cinema4D.
Command Line (Windows/Mac/Linux) - FBX to glTF: link
Command Line tool to convert models from FBX to glTF format. Great support for various animation types but can be tricky to use for people who are unfamiliar with using the command line.