Virtualisation

Virtualbox (os)
http://www.virtualbox.org/

VMware Workstation (trial)
http://www.vmware.com/download/ws/eval.html

VMware Player (free)
http://www.vmware.com/download/player/

VMware Server (free)
http://www.vmware.com/download/server/

VirtualPC
http://www.microsoft.com/downloads/details.aspx?displaylang=en

os: open-source; free: freeware [mostly for non-commercial use];
trial: evaluation version, mostly limited to 30 days;
share: shareware [1. free to use with all features for a short period of time, or 2. free to use with limited features - or nag-screens(winzip)], both kinds have to be paid to become the full versions