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Try it out: http://saintamh.org/apps/capitalist/
Source (plain): svn://svn.saintamh.org/code/capitalist/trunk/
Source (highlighted): http://svn.saintamh.org/code/capitalist/trunk/

I use this to monitor book prices on several online retailers' websites. The program has a list of books and DVDs I'm interested in buying, and once a day searches for these items on several websites. It then aggregates these prices, removes duplicate or uninteresting offers, calculates shipping fees, converts currencies, compares found prices to previous ones for the same book, and alerts me by email if it finds what it thinks is a good deal.

As of October 2010, the system is monitoring 20 retailers (and 1 library).