Unpublished manuscripts can be a a couple of things:
With many grant-funding agencies (like Canada's Tri-Agencies) requiring the outputs of research to be publicly available, researchers are depositing their works (or versions of their works) into publicly-accessible repositories. Everything from data sets, to manuscripts, and software code, can be available open access. Even if open access publishing is not required, more and more researchers are making a conscious decision to share open access versions of their works so that knowledge can proliferate more widely and not be hidden behind paywalls, speeding up the dissemination of research.
Another reason for searching for unpublished manuscripts has to do with the challenges of commercial publishing, which tends to favour novel, positive results. Searching for unpublished studies can help balance that by including studies with negative or inconclusive results, giving you a fuller picture of the evidence.
For more on positive results bias, see:
Plüddemann, A., Banerjee, A., & O’Sullivan, J. (2017). Positive results bias. Catalogue of Bias. https://catalogofbias.org/biases/positive-results-bias/
There are many preprint archives to choose from. Several discipline-specific ones are hosted on the Open Science Framework's Preprints Server (OSF Preprints), with the active ones listed below. Institution-specific repositories (like our own UVicSpace) are another source for pre- and postprints. Wikipedia maintains a list of preprint repositories which is also worth a scan.
Ask us