The announcement was made simultaneously with the latest release of the platform, Movable Type 4.0, which will continue to be available in both a commercial, proprietary license and as an open source, GPL version.
SixApart also said it has made improvements to TypePad, its hosted platform, and that it has released the beta version of Movable Type 4.0, the products first full point release since 2004.
SixApart said the announcements were the first of several coming this summer.
The latest version of Movable Type (MT), which SixApart is promoting as a "social media platform," includes more than 50 new features, including an installation and upgrade wizard, upgraded template management tools, new default templates and themes, and a completely redesigned user interface.
These improvements will help engage corporate customers unfamiliar with blogging and help SixApart position itself as enterprises begin to adopt many social networking applications that have become popular in the consumer space.
MT 4.0 also allows blog readers to become members of a site and gain posting privileges.
Perhaps most importantly for veteran bloggers, MT 4.0 includes 10 of the top 15 third-party extensions developed for prior versions of MT.
SixApart also said that TypePad now offers the ability to add static, non-blog pages, which is an important feature for users who want to grow their sites beyond blog communication.
The move to open source MT marks a dramatic shift in strategy for the company, which for several years enjoyed near celebrity status among bloggers, who praised the companys free products, and developers, who created hundreds of extensions for the software. The company lost its flavor-of-the-month status, though, when it decided to charge for version 3.0 of MT.
But SixApart Vice President Anil Dash said the companys motive was not to win back users upset at the companys pricing structure.
"If that had been our motive for open sourcing, we would have been doing that years ago," Dash said, arguing that the audience that cares about those issues is relatively small, mostly a few developers and tech bloggers. "We wanted to take advantage of our existing open-source work in LiveJournal and Vox."
Dash said that, besides the open-source code products LiveJournal and Vox, perhaps more than two-thirds of the code in MT 3.3 was also non-proprietary.
"Its easy for us to [make MT open source] because of the existing licensing structure we have in place with businesses," he added.
According to SixApart, MT powers blogs for the Washington Post, Huffington Post, General Motors, BoingBoing, Intel and millions of other businesses.
The MT 4.0 beta is available for download at movabletype.org. The personal use of MT is free and individuals can purchase tech support. Commercial, education and non-profit licenses for the current version of Movable Type, MT3.35, will include a discounted upgrade to MT4.0 during the beta period.