Free

Productivity addons
As an Industrial Designer, I regularly develop lightweight productivity extensions that I use daily. You can download each of them for free from Blender's Extension platform.
Paid

Edit Guide Mesh and Final Mesh Simultaneously
Do you model cars with the Shrinkwrap method? Are you in search of a more concept modeling-friendly approach? Have you ever thought I'd be great to edit both the Guide Mesh and Final Mesh simultaneously? If so, the GFM Editor is probably just what you need.
Free

Alias Matcaps and Shading + Pie menu
Ever missed the famous Alias ‘chrome’ shader? This simple tutorial explains how to achieve a similar effect in Blender.