WordPress is an open-source content management system for websites that I use very often for my personal projects and client work. In fact, it powers this site as well. Recently there was a very contentious and public spat between Matt Mullenweg (original co-developer and primary frontman for WordPress and CEO of Automattic) and Chris Pearson (developer of Thesis, a commercial WordPress theme). The argument was over whether Thesis should be at least be partially licensed under the GPL (the license WordPress uses). I’m not going to rehash all the stuff that was said but my opinion is that yes, WordPress themes and plugins are GPL.