Movable Type blog software has been released under GNU GPL license. From the announcement page:
As of today, and forever forward, Movable Type is open source. This means you can freely modify, redistribute, and use Movable Type for any purpose you choose. Like many of us on the team, some of you have been waiting for this moment for years.
For a business, an open source license affects boring things like how a product is created, updated, and distributed. But the open source movement has always been about something more important: Freedom. With a name like "Movable Type", we've always been keenly aware of the importance of freedom, as that name echoes both the birth of the printing press and the creation of independent media that an individual can control.