The Blender 3D modeler
Since SketchUp became a subscription product, I've been looking for a viable alternative for hobbyist CAD. I thought that SolveSpace might fit the bill, but it's not as easy to quickly sketch out furniture and rooms as SketchUp. After struggling for a while, I eventually realized that Blender, the veritable open source 3D modeling tool, could be extended to support a SketchUp-like workflow.
The key is the Construction Lines add-on, which makes laying out lines and then extruding from them almost as easy as SketchUp. There's a forum post that announces its features and a tutorial to apply them to a simple model. While Blender has a daunting learning curve, it also has a huge community with good learning resources.
For more technical machining or 3D printing needs, CAD Sketcher brings parametric CAD to Blender. I don't have much use for something like that anymore, but it's nice to have the option.
A few other plug-ins round out Blender for woodworking:
Cut List Creator paired with Cut List Optimizer take a model and generate the lumber and cuts needed to make it a reality.
Outline to SVG can export models for printing or plotting.
Carvature creates wood textures based on parameters, instead of just photos of wood.
As a bonus, Blender has industry-leading renderers to generate photo-realistic proofs.
Hollis Brown posted videos for various keyboard shortcuts at Blender Shortcuts. A numpad looks pretty helpful.