Using VMware Fusion and Multiple IEs, I set up my iMac to run the four major browsers simultaneously! IE 7, IE6, Firefox and Safari. With a 1920x1200 display, I can get all four to fit on the screen at once.
Click the image for the full-size version.
I no longer need to keep an extra Windows machine around here for debugging websites! "Multiple IEs" also can install IE 5.5, 5.1, 4 and 3, although I'm pretty sure those are almost completely irrelevant at this point.
One problem I did run into was that VMware in "Unity" mode didn't seem to play well with Spaces. For example, sometimes after switching spaces a Windows app would lose focus and I wouldn't be able to click it anymore. Switching to "Single Window" mode and then back again seemed to fix it.
Useful software:
