DisplayCAL, by Florian Höch, is based on the excellent ArgyllCMS open source calibration software (developer Graeme Gill - a Melbourne local!).
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:
Enable you to continue to use a calibrator where the maker of it has stopped supporting the device (e.g. with the latest operating system versions)
Enable you to use the more expensive features of calibrators that are reduced, software limited versions (e.g. use all the features of a Spyder5Elite when you only bought the Spyder5Express)
Enable you to use re-badged devices as OEM devices (i.e. remove the Eizo only limitation of the Eizo EX3 and use it as a gull Spyder5 device with any monitor)