Social software is networked software that allows communities to collaboratively build knowledge. As Joseph Ugoretz defines it,
Social software includes many communication media, but the new tools which are the subject of this essay all fit three broad descriptions. These tools are interactive, with the content created and structured by a wide mass of contributors. These tools are also interconnected, with user-provided searchable links structuring and cross-referencing that content. And finally, these tools are bottom-up and communitarian, with the users of the tools providing and benefitting from associations, reputations, and authority within a many-to-many community. (Emphasis added)
Social networking software provides many opportunities for gathering, customizing, and sharing information. But proceed with caution: since the information is user-generated and community-patrolled, it varies in accuracy and reliability. As with any other information source, you should compare it with other sources.
Here's a brief, incomplete list of social networking software. Did we miss one of your favorite resources? Put it in the comments.
Definitions and Descriptions
Images
Flickr. The popular tagged photography site.
MorgueFile. Public domain photography, suitable for web projects.
Video
YouTube. The popular tagged video clips site.
Google Video. Another tagged video site, including longer videos.
Tagging
del.icio.us. The best-known tagging service for bookmarking the Web.
Furl. Another tagging service.
Bibliographic Software
BibSonomy. "BibSonomy is a system for sharing bookmarks and lists of literature." It produces output that can be imported into EndNote or BibTeX.
CiteULike. "CiteULike is a free service to help academics to share, store, and organise the academic papers they are reading." It produces output that can be imported into EndNote or BibTeX.
Connotea. "Connotea is a free website to help researchers and clinicians manage and share information."
Also see this recent Metafilter thread on bibliographic reference management.
Calendars
Miscellaneous
Wizlite. "Wizlite allows you to highlight text (like on real paper) on any page on the Internet and share it with everybody (or just your friends)."
Diigo is a free web-based research and annotation tool. "Diigo is about 'Social Annotation.'" It includes plugins for major browsers.
YubNub. "A social command line for the web."
Wink. A social search engine.
Articles and Books on Social Networking
"Social Bookmarking" entry on Wikipedia.
"Hierarchies versus Facets versus Tags". An overview of the differences among three classification systems commonly used in social networking.
Tom Gruber. "Ontology of Folksonomy: A Mash-up of Apples and Oranges". Thoughts on ontologies, folksonomies, and how they can be productively used in social networking.
Benkler, Yochai. The Wealth of Networks. A book on social networking, free for download, with accompanying wiki for developing ideas.
Ben Lund, Tony Hammond, Martin Flack and Timo Hannay. "A Case Study - Connotea". A case study of a social bibliographic tool.
Spinuzzi, Clay. "The elitist argument against the web". A roundup of discussions for and against user-generated sites.
Merholtz, Peter. "Elite Design Agencies" and "Web 2.0". A discussion of design and Web 2.0, particularly in the context of marketing.
Ratliff, Clancy. "Social Bookmarking: Comparing and Contrasting del.icio.us, CiteULike, and H2O". A comparison of three social networking apps.
Shirky, Clay. "Ontology is Overrated--Categories, Links, and Tags." A comparison of taxonomies and folksonomies.