This is a hardware project I started when I was a senior in college. U2F Zero, a open source U2F token / two factor authentication device.
https://www.amazon.com/gp/product/B01L9DUPK6 https://conorpp.com/designing-and-producing-2fa-tokens-to-se...
Brings in around $500-$1000 per month of revenue (when in stock haha). I started just to learn how to lay out a circuit board and make my own embedded device, just kind of took off.
Using the U2F protocol is nice, since it's a standard and the PC side infrastructure is all there for the users to use it. I just have to do the hardware development :)
Also, I'm working on a new U2F token. It supports FIDO2 (password replacement protocol, upgrade to U2F), NFC, USB-C, and will have a nice case. If you're interested, sign up here, we'll be releasing more news soon :)