The main reason of almost every post on this blog is the same.
Need to do something, dig the Net, take a while to find stuff so I think to make a note on my… virtual hints book 
This time the need was to move a Windows 8.1 virtual machine from VMware Fusion for OS X 10.9.4 to VirtualBox on Linux.
The best option is to export and import the VM in OVA (Open Virtual Appliance) format, but there is no GUI option to export in Fusion. It took me a while to understand that Fusion comes with OVF Tool included, as a command line, and located in the default path “/Applications/VMware Fusion.app/Contents/Library/VMware OVF Tool”
A working example of a command line is:
./ovftool --acceptAllEulas /Users/marco/Documents/Virtual\ Machines.localized/Windows\ 8.1\ x64.vmwarevm/Windows\ 8.1\ x64.vmx /Users/marco/Desktop/Win81.ova
The green part of the command above refers to specific names I gave to the VM in Fusion and to the OVA file.
–acceptAllEulas is an option to make the command less interactive.
Enjoy.