Authors are typically allowed to post the "author-approved" final pre-publication version of their work online, which does not conflict with publisher copyright policies. SHERPA/RoMEO is a database of publishers' policies on copyright and self-archiving.
In many disciplines, particularly STEM, authors may choose to post preprints in order to avoid publication delays, despite online journals' making articles available immediately upon final acceptance. An article's presence on either an E-print repository or a Pre-print server does not guarantee that the article has been through the peer-review process.
The number of preprint servers is expanding, with several new initiatives recently announced. This list includes both established servers and emerging ones and will be added to over time.