Well, there is no such business without investment actually. It needs at least some money to start a small business. Some people say that there is a lot of business that doesn't need money to start but the harsh true thing is that every business needs some money if you want to earn money then :P
There is some business I know about that
Dropshipping: amazon automation service
around the world.
If you sell a product from a particular store or marketplace you will get a 4-10% commission. It is familiar with affiliate marketing. Big giant companies name as Amazon also support affiliates for their customer. You can also do this and earn a good amount of money from there.
Nowadays blogging is more popular. it's possible to earn from there. Also, you can earn from paid sponsorship from a restaurant or something you work for. There is some topic you should choose because this topic is trading and also it is possible to get more audience from the topic.
you can earn money from Facebook and youtube also by making some videos. These days, people are more used to seeing video more than reading a huge blog. So here is an opportunity to get some organic traffic and earn some smart money online.