Froogle.com, which now seems to have been renamed "Google Product Search" is Google's tool for searching ecommerce sites, and comparing any prices it finds.

On it's homepage is a random selection of recent queries. These are from real users - who will be ready to buy the products they find.

This can give you a very quick overview of the kind of products people are looking to buy, and hopefully give you some ideas to build a site around.

Of course, this should not be the end of your research - if you find a product you think you could build a site around (to monetize via Adsense or by selling the product as an affiliate), there are many additional things you can research first. Regardless, Froogle makes a great starting point for niche research!