One of the benefits of virtualization is the simplification of backup and recovery down to the “bare metal.” Bare metal recovery used to be a complex process requiring extremely similar if not the same hardware to restore to, or special software to make sure that the differing hardware didn't cause problems with restoring the operating system. That's largely a non-issue when restoring workloads back into the virtualized environment.
The trend in virtualized environments is agent-less backup which interacts with the hypervisor to quiesce the workload, take a hypervisor snapshot, and backup that snapshot. Essentially, the vSphere, Hyper-V, or XenServer native tool installed on the workload acts as the agent to quiesce the workload for an application-consistent snapshot. VMware vSphere, Microsoft Hyper-V, and Citrix XenServer have native APIs to allow backup software to interact with the hypervisor and workload agents for this process.
There are several software tools that focus on interacting with the APIs in question and adding their own value propositions as well.
Veeam Backup and Replication
PHD Virtual Backup and Replication
AppAssure Replay (bought by Dell Feb 2012)