VMware PowerCLI is a set of Windows PowerShell snapins that provide access to the VMware infrastructure just like the vSphere client. It has 165 commandlets. This will connect directly to hosts just like the vSphere client will, so it can be used to manager smaller installations. While very powerful scripts can be used for doing just about anything, here are some simple examples:
copying ISOs to and from the datastore
powering on or off machines
seeing how much space a machine is taking up
There are many scripts already written and available on the net, so search around before going to very much trouble to write your own. [more]
You can download PowerCLI from http://communities.vmware.com/community/vmtn/vsphere/automationtools/powercli. This page has several links, including a link to a getting started guide.