A few months ago I mentioned being impressed [1] with Enso [2]—enough that I was thinking of buying it.
Now I don't have to—Enso is now free [3] (as in beer at least; link via Compositing [4]). It's still Windows only, but maybe in time there'll be a version for both Linux and Mac OS-X.
[2] http://humanized.com/enso/
[3] http://humanized.com/weblog/2008/01/15/if-you-love-something-set-it-free/