Yes. Both Microsoft and VMware have made their software-based solutions free (as in beer), though I didn't get very good results running more than about 4 VMs on a server at once with either of them. Using ESX I get radically better performance and consolidation. Since the consolidated solution only requires 4 servers, though, you could potentially build the whole thing inside VMware Server / Microsoft Virtual Server R2.
As an added bonus, you can burn everything to DVD and make your lab completely portable, for presentation or testing.