IETester

IETester is a great essential tool for web designers. With this tool, you can know how the web page is displayed in IE6, IE7, or IE8 beta2. IETester v0.2.3 can test 4 IE browsers including IE5.5, IE6, IE7, IE8 beta2.
Display status in IE8, IE7 IE 6 and IE5.5 on Windows 7, Vista and XP.

Browser compatibility check for Internet Explorer versions from 5.5 to 10:
Over 4 million downloads.
IETester is free.
 
IETester is a free (both for personal and professional use) web browser that allows you to preview rendering and JavaScript engines with IE10, IE9, IE8, IE7, IE 6 and IE 5.5 on Windows 7, Vista and XP, as well as install the same IE browser in process. 
The new version can already support ie9.

IETester.exe is the main file.
Neither the previous/next buttons work properly
Focus is not working properly
Java applets not working
Flash does not work on IE6 instances in user mode: One workaround is to launch IETester as an admin user and have Flash work.
CSS filters are not working correctly in user mode: one solution is to launch IETester as an admin user and CSS filters will work.
windows.open does not return newly created window but empty (IETester limitation)

Update log:

April 2, 2013: v0.5.2
window.external bug fixes:
Fixed crash when adding to favorites (wrong parameters)
Set to "disable window.external" to work
Delete the window.external method in the pop-up window
March 12, 2013: v0.5.1
The bugfixed language is an instance of IE6.
February 10, 2013: V0.5
development. The current location of the bar:
on the left
IE tab at the bottom
floating
Added full screen mode
Added "Hide Ribbon" option (ribbon can be restored from system menu)
Added new command line options for full screen and hidden ribbon flags
Added new command line flag to maximize window on startup
Fixed a major bug in the IETester engine that should be more stable across IE versions.
Note: This version is compatible with DebugBar v7.0+ only