Opera is nice but doesn't work with many websites e.g. most online banking / financials.
Despite what is said, many websites are being developed for IE6 only and, more to the point, are only ever tested with IE6. There are too many features in the HTML produced by today's tools (esp. that piece of very popular junk called Frontpage) which show up differently on different browsers.
Perhaps a real pro can develop the sort of fancy website which most marketing people want and make it browser independent but nowadays there are more website designers than there were tree surgeons after 1987
For most non-geeks, IE6 is a necessity.