As you may know the to be tested software can depend on external service like SMTP or LDAP. Using the production environment is not the best idea, since there are very different goals on a goal sheet for an IT-guy and a tester.
Therefore someone (me) has to take care of it and provide all the required servers, desktop, services and tools.
To test our tape libraries we need e.g. SCSI, Email-, LDAP-, HTTP, CA-, vendor specific encryption key server like ESKM or SKLM, Active Directory, file servers and data to actually backup for the system integration with different Backup software vendors..
Almost every service can have a multitude of different configurations and performance for most of these systems is not the highest priority or if you choose the right version (Postfix vs. Exchange) they do not need a lot of resources.
A good way to share your certainly limited resources is virtualization.
This way it is easy to run like 6 Linux email servers with different security configuration with 1.5 GB RAM and 10 GB HDD including web mail. Others like the key servers need Windows and min 4GB RAM and you need like 3 Version and replication and and and... Or you need different version of IE or FF or Chrome
VMware, KVM, Xen are your friends.
Another big benefit of virtual machines are snapshots, you have a setup working server/desktop with all the needed tools an services. Freeze it with a snapshot and when a tester has managed to break it, simply reset it and it's working again.