Isotope may be used in commercial projects and applications with the one-time purchase of a commercial license. If you are paid to do your job, and part of your job is implementing Isotope, a commercial license is required.
http://isotope.metafizzy.co/license.html
For non-commercial, personal, or open source projects and applications, you may use Isotope under the terms of the [GPL v3 License](http://choosealicense.com/licenses/gpl-v3/). You may use Isotope for free.