Taking Aim

Comparison-shopping software lets businesses target potential customers when they're browsing for products on the sites of competitors. Here's how it works.

1. A shopper downloads the comparison-shopping software, which is often bundled with screen savers and other freebies.

2. The software follows the shopper as she surfs the Web, tracking search terms and URL addresses along the way.

3. If the shopper searches for, say, a digital camera at BestBuy.com, the software generates a pop-up box with links to vendors offering the same product for less.