Let me start by saying that I don't much like tag clouds. I find them quite horrendous actually.
I was looking around on this site, and there was this perfectly awful looking ball of utter chaos. I'm thinking "WTF IS that?"
It turned out to be a 3D tag cloud. It spins around, and the tag your mouse is over pops out of the cloud and gets all highlighty and stuff.
Of course you can't tell what the hell you're mousing over because it's such a friggin' mess, so it has no navigation value. Great gimmick value...for about 3 seconds.