FreeCAD - Free and open source CAD tool. Quite basic compared to other commercial CAD programs, but most of the features offered by those programs are very unlikely to be needed by amateurs or hobbyists.
OpenSCAD - This is a weird one. Rather than graphically editing your 3d models, like you would do in all other CAD programs, this program works more like a scripting language for writing your CAD as code, leaving the artistic aspects of design. It takes parametric modelling to its furthest extent.
Fusion360 - Commercial CAD offered by Autodesk. Free for hobbyists and students. Windows/macos only and forces online storage of your CAD. Many makers swear by this program.
Autodesk Inventor - Windows only. Has a student license. Very featured, but aimed more at professionals than hobbyists.
Hardware Supplier
McMaster-Carr - Your one stop shop for anything hardware. Everything you could find at your local hardware store plus more. Metal inserts for your 3d prints, fasteners for your anything, they will have it.