Windows

It’s a bit of a sad fact, but you can’t do serious or even amateur web development without handy access to Windows. I mean, obviously you’re using Mac OS X [wdt] for doing actual work, but you’ll always need to check your work with our good friend Internet Explorer 6, still the #1 browser on this here Internet.

For Windows on your Intel Mac, Parallels is a pretty good bet (Boot Camp is not, unless it’s on a second Mac, since you don’t want to go through the whole shutdown-startup thing all the time).

If you’re running a separate Windows box, use Synergy as a software KVM. It’s elegant and seamless, and also handles syncing your clipboard back and forth, which is invaluable.