Social software enables people to rendezvous, connect or collaborate through computer-mediated communication and to form online communities.

Broadly conceived, this term could encompass older media such as mailing lists and Usenet, but some would restrict its meaning to more recent software genres such as blogs and wikis. Others suggest that the term social software is best used not to refer to a single type of software, but rather to the use of two or more modes of computer-mediated communication that result in community formation. In this view, people form online communities by combining one-to-one (e.g., email and instant messaging), one-to-many (Web pages and blogs), and many-to-many (wikis) communication modes. In many online communities, real life meetings become part of the communication repertoire. The more specific term collaborative software applies to cooperative work systems.

Common to most definitions is the observation that some types of software seem to facilitate "bottom-up" community development, in which membership is voluntary, reputations are earned by winning the trust of other members, and the community's mission and governance are defined by the communities' members themselves. Communities formed by "bottom-up" processes are contrasted to the less vibrant collectivities formed by "top-down" software, in which users' roles are determined by an external authority and circumscribed by rigidly conceived software mechanisms (such as access rights).

The term also arose in the late nineties to describe software emerging out of alliances between programmers and social groups whose particular kinds of cultural intelligence are locked out of mainstream software. In this understanding of the term, the social is understood to also have a political and aesthetic sense, not simply acting as a kind of glue for a collection of normatively understood 'agents' whose inter-relations are formatted by software. What both positions share is an understanding that particular design decisions and the grammar of interactions made possible by each piece of software is socially significant. As the term has become more important to the computer industry, this earlier use of the term has often been edited out of memory

Tools for Online Communication
Instant Messaging
Internet Relay Chat
Internet forums
Blogs or Weblogs
Wikis
Social network services
Social network search engines
Social guides
Social bookmarking
Social Citations
Social Libraries
Social Shopping Applications
Peer-to-peer social networks
Collaborative real-time editing
Virtual presence
Virtual worlds and Massively-Multiplayer Online Games (MMOGs)
Other Specialized Social Applications

Adated from: http://en.wikipedia.org/wiki/Social_software