Hopefully you have some knowledges about the Type I and Type II Virtualization's difference and comparison.
VMware ESXi is very picky in the certified hardware list, it supports like branded/certified servers (IBM x3690 x5, x3650 M3, x3850 M5 or HP DL360 G7, DL380 G7, DELL R710, R910....)only. DIY hardware has to try your own luck.
In fact, VMware Workstation needs vaild purchased license (Trial license only 1 month), if you have license and legal concerns. You'd better get some fundings to purchase it.
Hyper-V R2 is a good choice ==> Microsoft Hyper-V is a free hypervisor-based virtualization system for x86-x64 systems. The stand-alone version of Hyper-V Server 2008 was first released as a free software together with availability Windows Server 2008. With the release of R2 version, the Hyper-V Server 2008 R2 is still available as the free software product.
Or Ubuntu 10.10 Desktop version + Oracle Virtualbox 4.0 ==> Various guest OS (Windows XP, Windows 7, Windows 2008 Server supported....) without legal issues.