/dev/random

This benchmark comparison from CNet Labs confirms my own subjective non-scientific observation: VMware is a lot faster than Parallels. I found that Parallels would sometimes make my old MacBook Pro freeze for as long as several minutes while starting up Windows, which didn’t happen with VMware Fusion. I haven’t installed Parallels on my Core 2 Duo MacBook Pro, so I don’t know if the performance would be any better.