A Dutch startup connecting a network of 3D printers (in both polymer and metal), CNC machines, and injection molders from around the globe to customers who need things printed, molded, and machined. https://www.3dhubs.com/ See also their 3D Printing Handbook (https://www.3dhubs.com/3d-printing-handbook)
A freely downloadable file to test and calibrate (benchmark) a polymer printer’s performance. http://www.3dbenchy.com/
The definitive source for comparative data on home-grade 3D printers. https://makezine.com/comparison/3dprinters/
This privately maintained database matches materials with needs (including technical specifications) and also materials with printers. If a builder wants to create something with specific strength, ductility, and other properties, he or she could enter the parameters, find the proper material, then also find the exact machines that can print it. http://senvol.com/database/
Wohlers Associates has published an annual report on the state of the additive manufacturing industry for more than twenty years, focusing on business issues (including mergers, acquisitions, and major product announcements), government (particularly standards bodies), and markets. https://wohlersassociates.com/wa.html