Just a couple of tools.  I ask about them because:

Orbiscope Tracker:
This looks like a pretty good tool:
all in all it seems to have most of what the other programs have, but two features I find of particular interest are:
a)  web page field tracker (e.g. stock price, product price / availability, etc.) and
b)  an image change tracker. 

Also does anyone know of a tool that will monitor the changes and combine this with extraction.  For example, if you're monitoring for a book list for availability and pricing and user rating from Amazon, and you only want to update your database when it does change?

Cheers,
--Malcolm.