One of the biggest annoyances of using Recon-ng is getting everything set up to use it. So here I'll outline the different API keys it can use and where to get them yourself. Some require you to pay money, which will be mentioned below. I suggest as you go along, you save all the API keys to a file so you can use them later.
Right now, these are the latest instructions for Recon-NG v4.8.3.
As more API keys are added (or Recon-NG version changes), I'll try to keep up with instructions on how to make the keys. Please feel free to submit a pull request or issue report for fixes or problems!