As a web application developer I spend a lot of time logging into sites as different users and prefer to be able to toggle back and forth between multiple instances of a browser maintaining the separate sessions in each instance. With IE 7 this was not an issue as long as IE was launched from the toolbar. In IE 8 sessions are shared by default, once again a Google search delivers the goods!
This post explains the -no-merge switch
as easy as Start -> Run ->
iexplore.exe -nomerge
FireFox has a similar issue, but the only work around I know of with FireFox is to create separate profiles, see this post - FireFox and multiple cookies for the same site