VMware specifies that if you're using a primary and recovery site, you'll need to be licensed for Virtual Center Manager Server at both locations. You'll also need VMware Infrastructure licenses for any server hosting VMware ESX—whether it's running or powered down.
If you're also using the Site Recovery Manager, you'll need licenses for any direction you have failover. For example, if you're configured to fail over in a single direction only, you need Site Recovery licenses for the primary site. If configured for bidirectional failover, you need to license hosts at both sites.
Citrix Xen has the same rules as VMware around licensing hosts, hot or cold. One difference: Citrix licenses per server, regardless of CPUs, whereas VMware licenses are per CPU.
With Microsoft, not surprisingly, the picture gets a bit fuzzy. If you don't have Microsoft Software Assurance, you need licenses for every server you have running, even cold servers. However, if your organization has purchased Software Assurance, one benefit is complimentary cold backup server licenses for purposes of disaster recovery. Servers must be turned off and used only in the event of a disaster. This applies to all Microsoft products, but it requires that you carry Software Assurance on the OS, application, and those pesky CALs, which can kill a budget surplus faster than a bipartisan vote.
As you move up the application stack, things get trickier. For example, you can have an Oracle Enterprise server license running on a failover server, but only if it accesses the same databases as the production server and is never on for more than 10 days—we kid you not.
As part of a DR plan, we highly recommend compiling a full software list and reviewing it with licensing vendors to ensure that you're compliant and not paying for licenses you don't need.
Need Disaster Recovery On The Cheap? Think Virtualization