Product review (4 stars): Not an official Honeywell packaged item, but works...
UPDATE:
A few years later now, I needed another replacement and finally got to the bottom of the problem I've been having with the filters I've been getting off Amazon: The photos are wrong!  The HAC-500 filter is square, and what I received was square, and it fits fine in my humidifier, but as you can see the photo is rectangular, and also because it didn't arrive in proper Honeywell packaging I was suspicious.  But now I do think it is a fully functional knockoff, and am adjusting my rating, and title accordingly...

Original:
Item is NOT from Honeywell, and NOT the right size. But it's too cheap to bother dealing with it so my best bet it to warn others...