Many people don't know about it - and it's almost like a huge secret, but Hyper-V is one of the most cost-effective Virtual solutions out there, and I'll tell you why.
In a single word - Licensing.
Microsoft give allowances for additional copies of its operating system for running in a Virtual Environment, and the breakdown is as follows:
The good news is that you are also licensed for lesser editions of Windows too! Scenarios of how you could utilize these license allowances are:
If you buy Windows Server Standard, then you can run Hyper-V inside it and install Windows Server Standard again. A good example of this would be your Host Server can be a Domain Controller, and everybody knows that Exchange server is not recommended on a Domain Controller, so you can install Hyper-V, and run Exchange in your Virtual Server.
If you buy Windows Server Enterprise, then you can run this server and host 4 machines inside. 1 x Windows Enterprise for your Domain Controller/Certificate Authority, 1 x Windows Standard for another Domain Controller, 1 x Server for Exchange and 1 x Server for File & Print.
You can also combine the licensing allowances - For example, if you have two Enterprise licenses, you can run 8 Virtual machines across 1 server, or mix and match between the 2 servers. 2 + 6, 3 + 5 - it's all fine.
And the Ultimate Money Saving Hair-Brained idea:
Buy two severs which have 4 x 10 core processors with 2 threads per core - giving you an effective 80 cores per server - shove in 1 Terabyte of RAM, and license it with Windows 2008 Datacenter edition. Naturally, you'd need 2 x 4 CPU licenses (you only license the physical CPU, not the logical ones), but you can run an UNLIMITED amount of Windows Servers inside these servers.
Why two? Redundancy of course, put the two in a cluster, so if one fails you don't lose your 200 free virtual servers you're running.
Don't forget the CAL's!
Regrettably, this licensing scheme does not include user or workstation CAL's, so you still have to buy those.