Not sure it’s the official name for this phenomena, but pretty descriptive.
StumbleUpon calls them tag clouds. I’ve been seeing these around lately, I like them even though they may not be extremely efficient they are nice eye candy and functional.
Would it be a good experiment for my own article index at thatgrrl.ca? It’s becoming a problem over there, kind of like a dinosaur come back to life and threatening to eat the whole village in a gulp.