Up until a few short months ago Movable Type, which is my preferred blogging platform, was not open source. While there was a free personal license if you made commercial use of the software you were honour bound to get a commercial license of some kind.
Since then MovableType has launched an open source project, while still maintaining its commerical versions.
If you want to play around with a robust blogging / CMS platform then you can either opt for the personal license or go down the open source route.
What’s the difference?
The difference is primarily in terms of support.
If you opt for a personal license you can upgrade to get commercial support for a nominal fee. You can also upgrade your license and install to one of the other commercial versions of Movable Type that offer a variety of extra features.
With the Open Source version you are pretty much on your own and have to rely on the “community” for assistance.
While the open source route might be the best and most attractive option for some people it probably isn’t for others.
Ultimately I’d recommend you weigh up the pros and cons and make your own decision.