What are the most useful free or cheap to obtain certifications or training?

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food handlers permit, security guard quals if your state requires them

want to be a plumber? paid training provided by the unions.

I got paid to get my behavior tech certification. It's not that useful though.

FEMA offers a variety of courses in emergency management-related areas that are free to take, and can be transcripted as credit through a MD community college for thirty or forty dollars a credit.

Mediation training. I signed up for a community mediation program through a local non-profit and they gave me free professional training. It's not a licensed profession in my state, so I can become a paid mediator if I want to.

Our pest control guy used to be a baker. He did an online course and now he mostly pest proofs houses. And he doesn't have to get up at 4:00 in the morning. When I asked about dealing with dead animals, he said he doesn't deal with them much.