If all your hardware is supported by Linux kernel 2.6.18, and your CPU doesn't support hardware virtualisation, and you will only be running NetBSD or Linux in your virtual machines, then Xen 3.0 (not 3.1 or 3.2) is very nice. Fairly easy to use, very fast, very lightweight.
However, if your CPU support hardware virtualisation, then go with with a Linux distro that uses kernel 2.6.24, and use Kernel-based Virtual Machine (KVM). With that, you can create virtual machines you can run *any* 32-bit or 64-bit PC-based OS (any BSD, any Linux, any Windows, etc). KVM is very easy to use, is very lightweight, is very easy to understand, and performance is very close to native (around 80% in most benchmarks). There's also paravirtual disk and network drivers available for Linux and Windows which give native I/O performance. For more info, check http:///kvm.qumranet.com/kvmwiki/
We're moving all our VM hosts to KVM. It's just better than Xen.