It is free, and quite simply one of the best open source software packages I've seen. It can be used as an adjunct to, or replacement for, the vendor supplied calibration software.
It is an excellent, thoroughly documented piece of software that is of interest to anyone with a monitor calibration device. It is vastly more feature filled than the software that comes with most calibration devices, and well worth exploring if you want to go beyond the software that comes with your calibrator.
In addition, DisplayCAL supplies its own (very stable) drivers for devices, which means it can do at least three amazing things: