I don't know about you, but I often find myself saying "I'd totally buy that if it cost $__ less." It happens to me all the time with items such as RAM, video cards, Windows Vista (joking), and the Amazon Kindle. Luckily there are two handy services to help keep tabs on prices for online goods.
The first, and newer of the two is called Waitable. It simply monitors the price of whatever items you put in either by URL, Amazon.com sales number, or UPC code. All you need to do is plug in what price you'd like to pay (a la Priceline) and it sends you an e-mail if the item is selling at or below your requested amount.
In addition to e-mail notifications, Waitable provides an RSS feed you can subscribe to, or plug into your favorite feed reader to avoid having to check your in-box. The service only works on Amazon.com at the moment, but after seeing what PriceProtectr (review) can do on price drops after you buy something, I can imagine seeing other online retailers making their way on there in the future.
The other service is called WishRadar, and has a larger emphasis on what you can do with the list of items you put together. You can create and share various wishlists ad nauseum. It also imports whatever pre-existing list of products you've got on your Amazon.com wishlist, along with a bookmarklet you can click to add any product you're looking at (on Amazon or Half.com) without having to copy and paste.
The one thing both of these services are missing is a way to view a product's life cycle to see if there have been any price drops or fluctuations in the past. For example, a pair of headphones I bought and use on a daily basis quite regularly jump from about $90 to $70 on Amazon, and neither of these services would tell you that. Likewise, predictive services such as Farecast, have created algorithms to tell what's going to happen to the future price of airfare--something that could potentially be automated with reoccurring life cycles of major electronic goods.