The following are the six best out of the box SharePoint features:
Alerts - allow one to set up notifications when changes are made to a document or list.
Checkouts - allow one to check out a document so that no one else can edit it while it's being worked on.
Views - allow one to create custom views of documents and lists in order to help one find the information needed more easily.
Full-text indexing - offers full-text indexing of any Office based documents, allowing one to search for content within documents.
People Search - allows one to find people within the organization.
Indexing outside of SharePoint - ability to index information outside of SharePoint, such as file shares or other application-based content sources.