For Internet Explorer 8 (download), we’ve made browser session handling a lot simpler.  For instance, say you want to have two Hotmail windows open, each logged into a different account.  Simply click the New Session item on the File menu, and a new browser window will open.  The new browser window will not share session cookies with the original browser window, so you can log into Hotmail (and most web applications) as a different user.  For command line junkies, you can run iexplore.exe with the -nomerge parameter. In contrast, if you click New Window, New Tab, or Duplicate Tab on the File menu, or click on a desktop shortcut, the new tab or window will share session cookies with the original tab.  This is called “session merging.”

Full Article: IE blog