IE 8 has an excellent feature called as New Session. This launches a new IE 8 window and now you can login to two Gmail account or 2 Facebook account simultaneously without requiring any add-on or software.
I had never know this feature until Shriram pointed this to me when I was trying to troubleshoot a problem with him. When asking a couple of people more 99% did not knew it existed. Now How about other browsers implementing it 😉