Internet Explorer 8 has lot of new features including new session option. This allows you to open multiple web accounts of same website simultaneously. For example you can view 2 or more Gmail, Yahoo mail accounts in the same Internet Explorer 8 interface.
To use this feature, goto file > new session and open multiple sessions of same web service in Internet Explorer 8. Each session will have own cookies allowing you to access multiple accounts at the same time. To open new session manually, goto start > Run and then type iexplore.exe -nomerge and click on OK. One more reason to install Internet Explorer 8, isn’t it? [via IE Blog]