There are many great marketing tools out there that you do have to pay for but there are also a ton of tools that are free that work amazingly well. Like video streaming tools, Whatsapp, the multitude of social media sites, photo editors and many others.
So what's one that you can't live without now that you've found it? It may help someone else who could really use it too! A couple of my favorites are Photoscape x and Screencastomatic. I also use the Facebook call feature every single day for business.