The only complaint I have about Shopobot is that it doesn't track eBay prices, but other than that it's a very handy site that will tell you when the best time to purchase a tech item is.
So far it's only tracking gadgets and games, but over time I can see Shopobot adding a whole range of categories, pulling in from online stores such as Amazon, Walmart, B&H, Buy.com and more.
From my brief tinkle with the site, it seems to be very useful (if you live in the US.) You can choose categories you're interested in, and choose to follow items of interest. Some items, like Apple products, don't change much, but as Shopobot also displays used items' prices as well, you do get some leeway there. [Shopobot via NY Times]