Small company benefits?
We are a small company, around 50 employees. We've used VMWare Server (and before that VMWare GSX Server) to consolidate hardware, simplify server maintenance, allow the introduction of alternative operating systems and dramatically boost the effectiveness, speed and reliability of our disaster recovery solution.
For hardware consolidation we have 15 virtual servers (Windows/Linux) running on only 3 Dell Poweredge servers. That's MS Exchange, domain controllers, file servers, web servers, print servers, etc.
Server maintenance is helped a great deal. The sysadmin can try ill advised "upgrade" ideas on an offline copy of the production server before trying it on the real thing. That's helped us avoid quite a few problems in the past.
We had no Linux servers before virtualisation. The skills to manage Linux running directly on the hardware for mission critical services just weren't there. Nor was the confidence (in some quarters) that Linux was suitable. Virtualisation has allowed us to introduce Linux gently. It's allowed a server to be built which the sysadmin could treat as a "black box", starting, stopping and backing up via the familiar Windows UI. These days (several years later) we have in house Linux skills and Linux running directly on hardware for our most important mission critical servers.
The benefit of virtualisation to our disaster recovery solution can't be overstated. We backup virtual machine folders on to disk and tape. Simple, fast, no expensive "backup agents" or other complexity required and can be restored onto any hardware. To restore our entire Exchange server from backup is just a case of dragging a folder in Windows explorer, compared to the previous hell of being faced with a "system level" backup tape and some hardware and weekend in the office.
I can't imagine being without VMWare.