Alpha blending, cast shadows and other effects on HTML elements such as tables, tablecells and images. (IE only)
Alphablending example:

HTML:
<IMG style="FILTER: Alpha(opacity=50)" src=http://www.funkelab.com/files/chemicals.jpg>
X-Ray example:

HTML:
<IMG style="FILTER: Xray" src=http://www.funkelab.com/files/chemicals.jpg>
More info on this subject here