Quoting from the VMware official announcement:
VMware, the global leader in virtual infrastructure software for industry-standard systems, today announced that its virtual machine disk format specification for defining and formatting virtual machine environments is openly available, downloadable and free of charge to encourage use by all developers and software vendors.
In addition, VMware is committed to supporting any other open virtual machine disk formats broadly adopted by customers and working toward converging on open standards in this area.
Software vendors like Akimbi Systems, Altiris, BMC Software, PlateSpin, rPath, Surgient, Symantec and Trend Micro are leveraging the VMware virtual machine disk format specification to develop value-added products for customer virtual infrastructure environments.
Additional information on the VMware virtual machine disk format specification is available at www.vmware.com/vmdk.