When building an OVF, you can specify a tag for VirtualSystemCollection that allows you to have multiple VMs to share the same base disk image, but any changes that the individual machines make are Copy-On-Write into a private disk area for each specific VM.
When you try to deploy images setup this way to EXSi, it complains Unsupported element 'VirtualSystemCollection'. It would appear that you need the commercial vCenter or vApp servers from VMWare to utilize this feature. (From what I've been able to grok so far)
Is there a way to do this through free software (Free like ESXi, or opensource)
The ultimate goal is that I want to have a single disk image that's used as a base - and to bring up a cluster of VMs that are then individually configured so that for a VM with a 500 meg disk, I only need '500M + (num_vms * delta_per_vm)' rather than '500M * num_vms'
An ESXi connected to a vCenter should support this via vApps but since you wanted a non-commercial solution, the closest things is to use VirtualBox.
The open source VirtualBox has multi-attach support to achieve this with different disk formats and it works very well. It also has special qcow, qemu copy on write disk support. Basically, you create a master disk and attach to multiple VMs. (Huge disk space saving.)
It can also happily import multiple VMs from a single OVA file with VirtualSystemCollection but unfortunately, it still requires manual intervention to tell VirtualBox that disks are shared after importing all the VMs. (Well, it defeats the appliance deployement in the first place...)
After creating master disk (or after deployment), attaching to multiple VMs can be done with GUI or with the following command:
VBoxManage storageattach "vm-name" --storagectl "sata1" --port 0 --device 0 --type hdd --medium base.vdi --mtype multiattach
For more information, see http://www.electricmonk.nl/log/2011/09/24/multiple-virtualbox-vms-using-one-base-image-copy-on-write/ and http://virtbjorn.blogspot.com.tr/2012/12/virtualbox-multi-attach-disk.html
If you really want to use VMWare ESXi, you can use data deduplication to achieve the same task on block level. (which is generally used with cloud hosting companies). You can see the deduplication success rates with open source tools here: http://opendedup.org/deduprates