There are several tools that allow you to run IE8 and IE7 simultaneously to develop and test for both the versions of the browser, however with the introduction of developer tools for IE8, developers can now test their work in both the browsers from within IE8 itself.
To run a webpage in IE7 mode in IE8, open Tools > Developer Tools or use the F12 key to launch developer tools for a open tab.
In the Developer Tools, click on the Browser Mode and select Internet Explorer 7 from the list, the webpage should now reload and behave like it would do in IE7.
Pretty neat feature, IE8 is definitely looking good, keep tuned to know more about IE8.