Linux system replication

[Bob Swanson]

Can anyone suggest products for replicating Linux sysetms?  Something like a
DoubleTake or vConverter product.  I suppose VMWare may have something, too,
since the targets are pretty much always virtual machines.
 
I'm pretty sure RedHat has clustering available, but that's not what I'm
lookng for -- need to snap shot a machine, keep it in cold state, and
activate upon certain circumstances.
 
Thanks.