In scenarios where you would not want to publish your paper into expensive journals, you can deposit them in these free repositories. Unlike proper paper submission, you need to associate yourself to an institution such as IEEE, university, etc. However, after long usage of free publications, I would recommend the following options.
There are more free repository available for use but generally I would stick to the list. The higher the list, the better my recommendation to you. Those that were not mentioned either has bad / a lot of grey area in their business operations OR simply I have not use it at all.
Normally for sensitive and frontier researches, you would not consider these free channels as your institution is already having its own publication channels. Some may even consider the research as trade secrets.
However, normally these free channels are great for your own researches or sharing data. For that purpose, you would want to:
Zenodo.org (https://zenodo.org) is an open-source digital library built upon Invenio that enables anyone (including non-institution associated individuals) to contribute into Open Science. It is funded by European Commission (EC) CERN division. All publications are indexed in OpenAIRE.
Open Science knows no border so anyone in the world can contribute into its repository.
Academia.edu (https://www.academia.edu/) is a paper sharing network for research community, allowing unconventional paper sharing across the globe. It was the first well-known network that every scholars and academic staffs to use.
ResearchGate.net (https://www.researchgate.net) is social network for scientific community, allowing any individual to publish any kinds of data and papers on your profile. It is privately funded by multiple investors like Series C venture capital and Bill Gates himself.
That's all about free repository for publications.