ArXiv

arXiv (pronounced "archive"—the X represents the Greek letter chi [χ]) is an open-access repository of scientific pre-prints. All documents are stored as PDF.
Importance.
Generally, all scientific journals have peer-review procedures. This process sometimes take several years. The time lag made by this process will easily cause delay to the scientific community. arXiv was made to avoid this delay.
Usage at Wikipedia etc..
Wikipedia and some journals accept citations by arXiv. Some academic search engines (such as Google Scholar) can help users to find arXiv pre-prints.
Scientific areas.
arXiv includes pre-prints from various scientific fields such as:
Similar websites.
Today, there are several arXiv-like websites in other areas.