VMware Workstation revived virtual machines on commodity x86 PCs by combining dynamic binary translation, direct execution and device virtualization before Intel and AMD added dedicated virtualization support.
Xen made x86 virtualization efficient by modifying guest operating systems to cooperate with a small hypervisor, proving paravirtualization before Intel VT-x and AMD-V made unmodified guests easier to run.
HotSpot made runtime behavior part of the compiler's evidence, interpreting Java first, profiling real execution, and then compiling frequently executed paths with aggressive optimizations that could later be undone.
Amazon EC2's 2006 beta exposed virtual machines through a web-service interface with rapid provisioning and usage-based billing, helping turn infrastructure capacity from a procurement decision measured in servers into an API-driven resource measured in instance-hours.
IBM's CP/CMS and VM lineage showed that one physical mainframe could present many isolated virtual computers, turning virtualization into a practical operating-system architecture decades before modern cloud computing.