Apps I use to get the job done
Since I do a lot of browser testing, I use VMWare Fusion to run any OS I need on my Mac. There are a few great virtualization options out there (Parallels, to name one) but VMWare seems to have a great focus on keeping up with the latest OS changes to ensure things run smoothly. Sure beats having 3+ computers on my desk… especially in the hot summer months.
Load up on RAM
Running a guest OS is resource intensive. If you can, install as much RAM as possible so that you can run each VM at a decent level.
Keep legacy versions of OSX
Sometimes those older versions of MacOS come in handy for browser testing or in replicating an issue a client might have on an older machine. With virtual machines, you can duplicate their environment down to the hardware specs.