For those who only use Internet Explorer, I would like to remind you that there are many browsers like firefox, google chrome, opera, safari and so on for opening web pages. You can log into one facebook account only in one browser. So, you can just download other browsers too and log onto different facebook account on each browsers.
Not just two, you can log into more accounts if you have more than two different browsers. Also, not just facebook but this trick can be applied to any website which requires signing up. You may try it on gmail, orkut, blogger, myspace, youtube or any such sites.