Qemu simulated systems are running on a 4-core machine and are normally suspended with SIGSTOP. When a user wants to log in to a system, it's resumed (SIGCONT) and locked during the session. Once all user sessions are over (all locks are gone), the system is automatically suspended again. Waking up a system takes less than a second - much less time than what it takes to boot up an operating system.
A wide variety of free operating systems are supported, including minix, openSolaris, different flavors of BSD and a few exotic systems.
Shell access is available upon request.