I have around 30 virtual machines running on a single tower server running ESXi. Solaris/x86, Windows XP, 7, server. A dozen different Linux installations. (Mostly used for software development, with a Jenkins-based continuous integration system building code across different platforms, spinning VMs up as needed).
Pretty much anything I could do with a rack of servers, I can do remotely with a bunch of VMs. I can access the console remotely, reboot, power-on, power-off virtual machines remotely. I can create a new VM and install an OS on it remotely. Add network switches, replumb the network between them. Mount or eject ISO images.
And there's stuff you can't do easily with physical servers that you can with VMs. Take a system snapshot, change something or test something, then roll back to the snapshot.
For "production" use there are a lot of tradeoffs between hardware and virtualization, but to play with or develop on it's hard to beat.
I have 8 cores, 16 gigs of RAM and a bit under 3 terabytes of disk. It cost a lot less, burns less power and makes *much* less noise than the rack of servers it replaced. You could get by with a lot less than that if you limited the number of VMs you had running concurrently.