XSIBackup

Try XSIBackup, you can download it at: Free VMware Backup solutions for ESXi OS download | SourceForge.net

It performs hot backups, so your VMs will always be available.
It also performs cold backups, shutting down your VM prior to the backup and turns it on when the backup has finished.
It is cron programmable.
Can automatically make room for your new backup in the backup device once it is full based on the full size of the device or a configurable space limit.
It is capable of using Rsync to perform differential backups, mirrored to a second ESXI box or to a local datastore.
Can overwrite backups or create a new folder for each new backup.
It's self contained, will run in the hypervisor without the need of additional hardware.
Will work on the free version of the ESXi hypervisor.
Will create and use it's own SSL key pair allowing to stablish trust relationships between different servers that will be preserved across reboots.
It's super easy to install with this one liner: wget http://33hops.com/downloads/?f=xsibackup.zip -O xsibackup.zip && unzip -o xsibackup.zip && chmod 0700 xsibackup*

Veeam Backup