Twitter is a free social networking and micro-blogging service, that allows its users to send and read other users' updates (also known as tweets). Now, you can build your own Twitter like service using trillr1.
From the announcement blog:
Trillr is a Twitter-like microblogging service enhanced with user directory and group discussion features. It was born in November 2007 as a peer group experiment where Jan Brauer, Christian Pesch and I decided to create "something like Twitter" to learn more about the programming language Python and its web framework Django.
You can download trillr1 source code here.