Parallels' killer feature is Coherence, a display mode that lets Windows apps and Mac apps seem to co-mingle with one another. VMWare, the market leader in virtualization, had a solid Mac offer (in beta) called Fusion but it lacked a similar feature. It looks like VMWare has figured out that people are crazy about Coherence, and so Unity is coming to a Fusion install near you.
As you can see in this video, Unity is pretty cool. It seems to have a little more polish than Parallels' implementation (the drop shadows are a nice touch) but I'll reserve judgement until I can use it myself.
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