According to StatCounter.com, IE6 — the bug-ridden, security-plagued, non-standard bane of web development — has fallen to an all-time low rate of fewer than 3% of all browsers in use. Thank goodness! It overstayed its welcome for about 5 years, long after IE 7 and 8 were introduced. For much of the last decade, developing web sites that would display properly in IE 6 as well as in browsers that actually worked properly often meant doing the work twice.
IE 7 was better, but still did weird things, and the further good news is that version 7 only accounts for about 13% of all browsers in use. IE 8 and IE 9 (now in beta) are W3C standards-compliant and display pages the same way as Firefox, Chrome, Safari and others.
You can help drive in the final nail by pasting a bit of code in all your pages that uses IE conditional comments to tell people using the old version that they should upgrade. Get the code at www.ie6nomore.com.