Wikipedia:Citing sources
User:Richiez has tools to automatically handle citations for a whole article at a time. Converts occurrences of {{pmid|XXXXXXXX}} (PubMed) or {{isbn|XXXXXXXXXX}} to properly formatted footnote or Harvard-style references. Written in Ruby and requires a working installation with basic libraries.
pubmed2wikipedia.xsl an XSL stylesheet transforming the XML output of PubMed to Wikipedia refs.

Reference management software: Reference management software can output formatted citations in several styles, including BibTeX, RIS, or Wikipedia citation template styles.

Comparison of reference management software – side-by-side comparison of various reference management software
Wikipedia:Citing sources with Zotero – essay on using Zotero to quickly add citations to articles. Zotero (by Roy Rosenzweig Center for History and New Media; license: Affero GPL) is open-source software with local reference database which can be synchronized between several computers over the online database (up to 300 MB without payment).
EndNote (by Thomson Reuters; license: proprietary)
Mendeley (by Elsevier; license: proprietary)
Paperpile (by Paperpile, LLC; license: proprietary)
Papers (by Springer; license: proprietary)

See also: How to cite

Wikipedia:References dos and don'ts – a concise summary of some of the most important guidance on this page
Help:Referencing for beginners – a simple practical guide to getting started
Help:How to mine a source – case study on getting maximum information from cited material
Wikipedia:Verification methods – listing examples of the most common ways that citations are used in Wikipedia articles
Wikipedia:Improving referencing efforts – essay on why references are important
Wikipedia:Citation templates – a full listing of various styles for citing all sorts of materials
Wikipedia:Citing sources/Example edits for different methods – showing comparative edit mode representations for different citation methods and techniques
Wikipedia:Citing sources/Further considerations – additional considerations for citing sources
Wikipedia:Inline citation – more information on inline citations
Wikipedia:Nesting footnotes – how-to guide on "nesting" footnotes
Wikipedia:Manual of Style/Layout § Further reading – for information about the "Further reading" section
Wikipedia:External links – for information about the "External links" section
Wikipedia:Plagiarism § Public-domain sources – guideline covering the inclusion of material in the public domain
Wikipedia:Scientific citation guidelines – guidelines for dealing with scientific and mathematical articles
Wikipedia:WikiProject Resource Exchange/Shared Resources – project guide on finding resources
MediaWiki:Extension:Cite – details of the software which support the <ref> parser hooks
Citation problems

Template:Irrelevant citation – inline template to note source simply is not relevant to the material
Template:More citations needed – template to add to article (or section) where citations are insufficient
Template:Text-source – template to add to article (or section) where text–source integrity is questioned
Wikipedia:Citation needed – explanation of template to tag statements that need a citation
Wikipedia:Citation overkill – why too many citations on one fact can be a bad thing
Wikipedia:Copyright problems – in case of text that has been copied verbatim inappropriately
Wikipedia:Link rot – guide to preventing link rot
Wikipedia:More seasoning doesn't mean more flavor – an essay about how less detail doesn't always mean less info
Wikipedia:You don't need to cite that the sky is blue – an essay advising: do not cite already obvious information
Wikipedia:You do need to cite that the sky is blue – an essay advising: just because something appears obvious to you does not mean it is obvious to everyone
Wikipedia:Video links – an essay discussing the use of citations linking to YouTube and other user-submitted video sites
Wikipedia:WikiProject Citation cleanup – a group of people devoted to cleaning up citations
Wikipedia:Reference database – essay/proposal
Changing citation style formats

WP:CITEVAR

Notes: 

Further reading: "Online Style Guide". New Oxford Style Manual. Oxford, UK: Oxford University Press. 2016. ISBN 978-0-19-876725-1.
The Chicago Manual of Style (17th ed.). Chicago: University of Chicago Press. 2017. ISBN 978-0-226-28705-8.
"Academic Writing: Citing Sources". Writers Workshop. University of Illinois.
"Citation Style Guides & Management Tools". Library Guides. LIU Post.
"Citing: Help & how-to". Concordia University Library.
"Citation Help". Subject Guides. University of Iowa.
"Guide to Citation Style Guides". Journalism Resources. University of Iowa.
"Library: Citing Sources & Citation Generators". Capital Community College.
"Research and Citation Resources". Online Writing Lab. Purdue University.
"The Writer's Handbook: Documentation". Writing Center. University of Wisconsin–Madison.
"ACS Style Guide". Research Guides. University of Wisconsin–Madison.
"Samples of Formatted References for Authors of Journal Articles". MEDLINE and PubMed: The Resources Guide. United States National Library of Medicine. 26 April 2018.

External links: 
"reFill". Toolforge. WP:ReFill. – tool that expands bare references semi-automatically
Wikipedia editing basics: Citing sources (part 1) (YouTube). Wikimedia Foundation.
Wikipedia editing basics: Citing sources (part 2) (YouTube). Wikimedia Foundation.