Preprint repositories have become very popular in recent years across many disciplines. Some may be restricted only to preprints while others allow a variety of materials to be deposited. Most include the option to add a link to the final published version of an article at a later date. For a brief explanation of preprints, see Preprints: The What, The Why, The How.
Opinions differ but here are the definitions from Sherpa/Romeo:
"This listing characterises pre-prints as being the version of the paper before peer review and post-prints as being the version of the paper after peer-review, with revisions having been made.
This means that in terms of content, post-prints are the article as published. However, in terms of appearance this might not be the same as the published article, as publishers often reserve for themselves their own arrangement of type-setting and formatting. Typically, this means that the author cannot use the publisher-generated .pdf file, but must make their own .pdf version for submission to a repository.
Having said that, some publishers insist that authors use the publisher-generated .pdf - seemingly because the publishers want their material to be seen as a professionally produced .pdf that fits with their own house-style." (Sherpa/Romeo definitions)
There are repositories for specific disciplines and some that are multidisciplinary. Check out the (not comprehensive) list of repositories on the Research Impact guide. New ones are cropping up all the time.