Almost anything involving computers, the internet, programming, or data. Every white collar job in the world revolves around them, and yet no one knows how to do anything with them which means $$$. Plus, at least to me, it's all interesting as hell too. There's just so much free or cheap information around it all too, load up on $10 classes on Udemy when there's a sale and you start there. The problem is though, it takes time to really learn all of this stuff and commit it to memory. Months to learn and a lifetime to master.