VIX
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+
VMware VIX 1.1 API reference Guide
VMware VIX 1.6 API reference Guide
vmrun commands (see also VIX API link above)
Whitepaper: Migrating From VIX API to the vSphere Guest Operations API
Presentations
VIX API Tech Talk Partner Exchange