Here is my most underrated tool that I use all the time.

Hardwood wedges. I make these on the table saw from scrap wood. Use them for felling trees, holding round logs in place on the sawmill, shimming posts or beams level and plumb, wedging warped boards into place, stealing brace boards, etc.

I keep a couple in every tool box and every truck, use them til they break, and then make some more.