The four things you often hear about are citrus juice, baking soda, distilled (non-fossil) vinegar and (potassiom saponified) soap. Be careful about mixing them because you don’t want to cause any weird chemical reactions. If you want to mix two, look it up on the internet first.
Also. Don’t be overly smug about how great these natural cleaning supplies are. Frankly, they’re kind of frustrating, it’s really hard to get things clean with them sometimes. Not that I’d know whether some proprietary, complicated chemical solution would do a better job, because I want to understand the things that I use at home.