I want to backup my virtual machines while they are running, is it possible to do this without buying more VM-Ware products?
They are running on CentOS 5.2.
It's been a long time since I have messed with vmware server (ESXi is free! :D), but I think you should be able to do it like this:
# vmkfstools -e /myVM2/target.vmdk -d vmfs /myVM1/source.vmdk
If you decide to run ESXi in the future, there is the fabulous ghettoVCB: http://communities.vmware.com/docs/DOC-8760
With vmware server ... no. With ESX yep using either the (now depreciated) VCB product, or any one of the commercial products that use the vSphere API, or you could write something against the API yourself.
I'm assuming you are using vmware server due to:
They are running on CentOS 5.2
Don't forget that you could also use any normal OS backup software like bacula or amanda to perform the backups from within the OS.
If you have an ESX(i) machine, you could use the vmware converter product to back them up.
Essentially, you point vmware converter at the guest VM's IP address, give it root login credentials, and tell it which ESX(i) server you want it to appear on. Then hit "go" and wait a while.
I've been very impressed with how reliably it's converted my machines. The only limitations appear to be on older machines that aren't supported (for example, a Win2k machine that hasn't been updated since the dawn of time didn't work, but would have if I could have updated it. Long story)
In ESX, it's possible to make snapshots, then manually copy certain disk files that aren't locked, in order to create a new machine. I'm not sure if you can do this in VMWare Server but it may be worth looking into.
ESX process: http://smkelly.livejournal.com/66017.html
Since you don't list any requirements for backup, I'll suggest
dd over ssh.
I use Trilead Vm Explorer: it's free, easy and works.
Another silly trick I use is to boot the virtual machine with a clonzilla cd and... Clone it like I would do with a physical pc but this would need a reboot so I think you are not interested :)