Firefox is an open-source browser from the Mozilla foundation. It has several advantages over Internet Explorer (security, privacy, features, easy to use user interface, etc). It is still in beta state, but functioning very well already.
Firefox can be downloaded here (Windows; Mac; Linux):
http://www.getfirefox.com/
Reasons for switching to firefox:
http://www.switch2firefox.com/
I have been using firefox for some time now. It is still in development, and starting the browser on slow computers takes a long time (compared to IE). But it offers superb features (the tab-browsing is really nice
) and also does well on security issues.