The Extrude, Inset, and Knife tools are three of the most important ones in Blender!
Extrude is used to create new geometry by extending the selected faces, edges, or vertices. Inset is used to create a new face inside the selected face, while the Knife tool is used to cut edges or vertices on a mesh and create new geometry.
The hotkeys you'll want to remember are:
When inside the tool's mode, you'll get several other hotkeys that you can use to change the tool's behavior, either in the status bar or in the tool header.
