Xyle scope for free. Not sure when this happened. Used to cost $20. I'm guessing this choice has a lot to do with the great success of Things. I wrote about Xyle scope in Jan. 2008. My conclusion then:
I almost bought this application but, in the end, I decided to stick with two free tools that perform most of the same feats as Xyle, even though I think they are much less elegant. I use Firefox when I'm working on websites, and have grown to rely on Chris Pederick's Web Developer and Joe Hewitt's Firebug.
I'm definitely adding Xyle scope to my toolbox now.