UPDATE: I wanted to pull this post out of the archives and update it a bit because it there seems to be a good amount of interest in this subject.
Here is what each of those CSS properties is for:
opacity: 0.5; This is the “most important” one because it is the current standard in CSS. This will work in most versions of Firefox, Safari, and Opera. This would be all you need if all browsers supported current standards. Which, of course, they don’t.
filter:alpha(opacity=50); This one you need for IE.
-moz-opacity:0.5; You need this one to support way old school versions of the Mozilla browsers like Netscape Navigator.
-khtml-opacity: 0.5; This is for way old versions of Safari (1.x) when the rendering engine it was using was still referred to as KTHML, as opposed to the current WebKit.