I've used both VMware Server and VirtualBox. I am using for now VMware Server 1 because the first beta of VMware Server 2 was a 'sloth'. VMware Server 1 does not support USB 2.0, my only complaint.
I'll be upgrading from 32bit to 64 in June taking openSUSE 10.3 to an AMD64 compile of openSUSE 11.0. At that point, I'll install VMware 2.0 beta 2.
VirtualBox is definitely a contender. Correct me if I am wrong Jason, but it doesn't support 64bit guest Windows operating systems.
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