Stop the press!
Joe Maddalone has stumbled upon something that could easily be one of the most important helpers to all web developers.
For years we were able to test our work in various browsers while developing, simply installing them into different directories. For all but Internet Explorer on Windows, which allowed only one version to run.
As of now, this is history.
Joe has a whole write-up on his page how he managed this.
Update: Joe is enhancing his explanations, as well as adding tutorials. I removed my feeble attempt to make a backup copy of instructions as new informations appear daily.