If you have an interchangable lens SLR camera and have been wonding about the quality difference between different lens, then check out this site. They rate each lens on a scale from 1 to 5. A typical score is 3.48.
Update: Also check out the lens tests at photodo.com (thanks for the info Uwe at outbackphoto.com)
The classic other lens test site is photodo.com:
http://photodo.com/nav/prodindex.html
Really good lenses are >4 and below 3 you shozld forget.
Uwe