http://twitter.com/– Maybe most popular micro-blogging service out there. I’ll be surprised if you don’t use it. So much is written about Twitter that I can’t add noting new.
http://friendfeed.com/ - FriendFeed is a micro-blogging service and feed aggregator that makes it easy to share information like messages, photos, videos and so on online. If you want to keep track on multiple feeds and share information with your friends, FriendFeed is your option.
http://identi.ca/ - Micro-blogging service based on the StatusNet tool. It is getting more and more popularity.
http://www.jaiku.com/ - Conversation-based service, currently owned by Google.
http://www.ping.fm/ - Free social network and micro-blogging service, that enables users to post on multiple social networks simultaneously.