If you need a free, open-source alternative for backing up your virtual machines, look no further than XSI Backup. Before I did an upgrade to some VM's I wanted to completely back them up (not just a snapshot), so I downloaded XSI Backup here: http://sourceforge.net/projects/xsibackup/
Once downloaded, copy it to the /opt folder on your ESXi host. Then you can run it and backup to a datastore that ESXi has mounted. For example, I want to backup all my VM's to my NFS datastore, I run:
/opt/xsibackup --backup-point=/vmfs/volumes/nfs-datastore/backups --backup-type=running
Running that command will backup all the VM's in real-time to the datastore. You can run
/opt/xsibackup --help to see all the variables you can specify, including backing up specific VM's and sending email alerts.