Works in IE 5.5+ and in all major browsers for years and is safe to use
Definitely the most used combination of CSS and IE’s filter. I mostly use it to make the website almost invisible in favor of an absolute placed div which floats like popup window on top of the greyed out website.

Opacity 0.5 in IE8

Opacity 0.5 in IE8

Opacity 0.5 in Firefox 3.5.5

Opacity 0.5 in Firefox 3.5.5