VIX allows you to automate virtual machine operations and is the engine behind for example VMware's vmrun command.
Supported on VMware Workstation 6.0.x and up, VMware Server 1.x and up, VMware ESX3.5U2+
The link for the API page no longer offers a working download (strange)
These however works, but you do need to login:
However you might have problems installing.
If you do then it is probably better to install VMware Workstation Professional which bundles VMware VIX and installs it on your behalf.
VMware Workstation has VMware Player as well and Player will continue to work after the Professional trial runs out.