There are many free-to-use programs available for designing and seeing other’s designs in 3D:
Tinkercad
is a free-of-charge, online 3D modeling program that runs in a web browser.Thingiverse
is an online community where you can share your own 3D designs, and browse others in their community.OpenSCAD
is a free software that focuses on the computer assisted design aspects of 3D modelling. Ideal for creating 3D models of machine parts.Ultimaker Cura
is a free software that will allow you to scale down and reprocess STL files before submitting them.