There are a number of significant vulnerabilities in technologies relating to the IE domain/zone security model, the DHTML object model, MIME type determination, and ActiveX. It is possible to reduce exposure to these vulnerabilities by using a different web browser, especially when browsing untrusted sites. Such a decision may, however, reduce the functionality of sites that require IE-specific features such as DHTML, VBScript, and ActiveX. Note that using a different web browser will not remove IE from a Windows system, and other programs may invoke IE, the WebBrowser ActiveX control, or the HTML rendering engine (MSHTML).
Firefox (Win/Mac), Mozilla (Win/Mac), Opera (Win/Mac), Camino (Mac only), Safari (Mac only) are good alternatives. I use FireFox on PCs, and Safari on my Mac. FF and Safari have a lot in common. Apple is tying Safari improvements to the operating system, so when I stop upgrading my aging iBook I expect I'll switch to using FireFox on both platforms.