Most of the VM players are offering free versions of their products, mostly aimed at developers and small businesses.
They typically lack the enterprisy features (High availability, resource allocation, fine permissions models) and scalability (clustering), but are nevertheless running the same core virtualization technology as the commercial products.
First, decide if you want a virtualisation product that can run on top of your existing workstation installation, or if you have some dedicated hardware that you can install a 'bare metal' hypervisor onto. The latter will net you higher performance, but there's perhaps a steeper learning curve and the prerequisite that you need some spare kit.
For the 'on my workstation' option, some candidates are:
- Sun VirtualBox
- Microsoft VirtualPC
- VMWare Player or Server
For the dedicated server, there's:
- VMWare ESXi
- Citrix Zenserver
- Microsoft Hyper-V Server 2008
These lists are by no means complete. Personally, I'd recommend Sun VirtualBox for ease of use and general straightforwards usage. ESXi is fantastic if you want something a bit gruntier. ESXi is also a more valuable learning tool if you ever come to work with full-blown VMWare vSphere in the future.
Most of the above products come with some roll-back feature built in (in VirtualBox and ESXi it's 'snapshots'). For backups, shutting down the VMs then backing up the hard disk files is a simple but effective measure.