I've always felt dirty using Vmware. First, it's not open source. Second, it just really feels like a bad fit. The architecture is very complicated. Server wants me to trust its security model. Also it depends heavily on my local libraries and precludes me from upgrading my Gnome or GTK installation. There are command line tools for manipulating the configurations, but they are hard to use and don't really seem like a very supported interface. Then, there is the plans for new Vmware Server which relies exclusively on a Java component to interact with the virtual machine. It seems far more complicated, brittle and harder to understand. So, I started looking for an alternative.
I've settled on Virtual Box. The major downside is that it does not support 64-bit guests. It's much simpler than Vmware; it doesn't depend on a network IPC mechanism and it uses the OS's security model. It's mostly open source: there is an open source version and there is a version with a few more features that is not open source. On one machine I can use the open source version; on another I need the USB support from the closed source version. It has kind of a rugged user interface, but one that meets all my needs. I seem to be very happy. Thanks to Marc Horowitz for pointing it out.