Microblogging
Twitter
A micro-blogging site that asks you to answer the question "What are you doing?" in 140 characters or less.
Tweetdeck
A free program designed to help you use Twitter more effectively.
Twitterfeed
A free service that will Tweet your latest blog posts.
Twitter Search
A search engine that allows you to search for Tweets pertaining to a specific topic.
Tagalus: Directory of Twitter Hashtags
A service that allows Twitter users to define and set Hashtags.
Plurk
A micro-blogging site that allows you to send brief (140 characters or less) updates, called Plurks, to a list of users who have chosen to follow you.
Jaiku
A Google run micro-blogging service.
Tumblr
Post anything quickly and easily—incorporates elements of a blog, a social network profile, video sharing
Yammer
Microblogging for organizations
Seesmic
A website that allows you to manage your social media profiles from one location.