If you go here: http://en.wordpress.com/tags/
You get a linear tag cloud, but here's the interesting part:
It only includes recent activity, so it should change quite a bit over time.
Now. I can't find, anywhere, a plugin that will let me achieve this goal. It's basically like trending terms on Twitter, and I'm staggered that a plugin hasn't been made for it.
Standard tag clouds are boring and pretty useless, but a tag cloud which displays the most popular tags from a recent time period is both useful and informative.
So, WordPress. Being as you've demonstrated that it can be done, how's about you let the rest of us in on the secret? :D