VMware Player is the freely available cousin of commercial app VMware Workstation.
The difference between them is similar to that between the full version of Adobe Acrobat and the Acrobat reader; VMware Workstation allows you to create virtual machines whereas Player only lets you use virtual machines that others have made and distributed.
This might seem a major drawback in that you can't, for example, run any versions of Windows or Mac OS as guest operating systems using Player since nobody can legally distribute virtual machines including these operating systems. However, if you're interested in Linux or Unix variants, there's no shortage of virtual machines.
We installed the 48MB DSL (Damned Small Linux) and it worked perfectly. It's also pertinent to point out that we experienced no noticeable degradation in performance – something that might be expected of the virtualisation approach.