Install multiple versions of browsers IE/Safari in the same environment
With the latest push from Microsoft making IE7 a critical update many web designer/developers are hesitant to install it in fear of losing IE6 compatibility (because the installation of IE7 overwrites your existing version of IE6).
update: the old tredosoft package breaks with IE8 installed. http://finalbuilds.edskes.net/iecollection.htm should get us back to where we were in the IE7 days with working form fields and conditional comments.
downloads for standalone versions of older Safari builds is here (easier than keeping an old one and using a recent webkit build): http://michelf.com/projects/multi-safari/
In the May 19, 2006 CWP meeting
(http://cwp.ucla.edu/presentations/2006/20060519/) a presentation briefly went over the use of multiple standalone versions of IE that could be downloaded and run on the same machine. These versions can still be downloaded via Evolt.org (http://browsers.evolt.org/?ie/32bit).
There is an old dev brief on the subject that also looks into virtualization and other solutions.
You can also get a premade XP VM image from Microsoft that includes Internet Explorer 6 for testing.