The answers in this post have grown dated. Although more recent, my answer is a summary from barely skimming the surface of this topic. Hopefully it will jumpstart updates to the thread and I have marked it Community Wiki in hopes that it evolves to better content.
XCP -> XenServer
As I have dug into this, I am increasingly finding that what was XCP has become the open source XenServer, as described at the Xen Project FAQ on this topic. Having now installed the Xen hypervisor twice, I'll test out the XenServer iso, which sounds like it may be the better comparison for most users (although I'm pretty happy with the bare hypervisor).
I have been interacting with systems hosted by a group I work with where they migrated in the last few years from VMware's free vSphere/ESXi to the commercial ESXi with vCenter. I have been helping them rehost servers into VMware run appliances. I have been impressed with the usability of the VMware solution and I see that this is attractive to the administrator, but there is a price tag for the enhanced feature set.
Having learned more about the free intro version, I am tempted to install that and see how user friendly it is, as I have one group that is familiar with Desktop VMware, but seems wary of server VMs, and this might be a good starter system for them.
Xen Project (hypervisor only)
To test out the Xen hypervisor, I went over to the Weird Stuff Warehouse, bought a used Dell rack-mounted server, and installed Xen (Xen Project, not Citrix). I primarily followed the Xen Project Beginners GUide and used the debian.org instructions as a reference. I stopped at the end of the PV (Paravirtualized) Guest instructions and I initially created three guests -- all Debian. For the lightweight servers I have in mind, Xen-based PV guests seem easy and more than adequate.
I originally planned to back up the VMs and then attempt to migrate the entire server setup to SW RAID (I found instructions at http://wiki.kartbuilding.net/index.php/RAID_1_and_Xen_(dom0)), but now I'm leaning toward testing out XenServer and/or VMware free products (open source in case of XenServer).
If you have a representative system, some time, and you enjoy experimenting, then try out a few of the free options. The hardest thing for me so far was locating USB media I could format - ended up using a spare SD from my phone with a reader - and finding the instructions for loading an iso onto USB media (Ubuntu has instructions for Ubuntu, Mac, and Windows.)