Publication of manuscripts in a peer-reviewed journal often takes weeks, months, or even years from the time of initial submission, owing to the time required by editors and reviewers to evaluate and critique manuscripts, and the time required by authors to address critiques. The need to quickly circulate current results within a scholarly community has led researchers to distribute documents known as preprints, which are manuscripts that have yet to undergo peer review. The immediate distribution of preprints allows authors to receive early feedback from their peers, which may be helpful in revising and preparing articles for submission (see Wikipedia: Preprint) 

• bioRxiv.org - the preprint server for Biology [https://www.biorxiv.org/]

• bioRxiv - the preprint server for biology, operated by Cold Spring Harbor Laboratory, a research and educational institution.

• medRxiv.org - the preprint server for Health Sciences [https://www.medrxiv.org/]

• medRxiv - the preprint server for Health Sciences, operated by Cold Spring Harbor Laboratory, a research and educational institution.

• engRxiv.org - the preprint server for Engineering - [https://engrxiv.org/]
• A new open archive of preprints in engineering, directed by a steering committee of engineers and members of the engineering librarian community.

• arXiv.org - [https://arxiv.org/]

• arXiv is an open-access repository of electronic preprints approved for posting after moderation, but not full peer review. 

• TechRxiv: IEEE Launches New Preprint Server [https://www.techrxiv.org/]

• IEEE recently launched TechRxiv (pronounced “tech archive”), a beta version of a new preprint server which is a free, publicly accessible archive, and repository for unpublished research or preprints in electrical engineering, computer science, and related technology.
• TechRxiv aims to increase the accessibility of scientific findings, enhance collaboration among researchers, improve research, and help build the future of scholarly communication. Authors can post preprints to TechRxiv regardless of where they eventually intend to submit and publish the preprint. Final versions of published articles will not be accepted or posted to TechRxiv.

• Research Square [https://www.researchsquare.com/]
• Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. You can also learn about breakthroughs in your field and comment on new discoveries.

• Asian Education Preprint Repository - [https://preprint.journal-asia.education/index.php/repository]

• Preprint Server For The Education Research Community [https://edarxiv.org/]

• Multidisciplinary Preprint Server [https://www.preprints.org/]