* Blogs: Short for weblog. Users post informal journals of their thoughts, comments, and philosophies and reflecting their views.
* Chats: Places on the Internet where people with similar interests "meet" and communicate together by typing instant messages.
* Email: Electronic mail system that can be used to send plain text or text with attachments.
* Discussion boards or forums: Online discussion groups, where participants with common interests exchange open messages.
* Instant messages: Programs that instantly send messages from one computer to another.
* Podcasts: Derived from Apple’s iPod, it is a method of publishing audio and video files to the Internet.
* Wikis: Allows users to freely create and edit Web page content and hyperlinks using a web browser. Wiki comes from the Hawaiian word ‘wiki wiki’ meaning fast. The most famous wiki is wikipedia.
Here is a YouTube video - "Web 2.0 - Catch the Wave" - in which Percy Parakh talks about some of the social software tools mentioned above.
More on Web 2.0.