Retargeted ads can draw visitors back to your website -- as long as they don't mind a bit of online stalking. Here are a few of the companies that let you advertise to Web visitors after they have wandered off.

AdRoll
This company, which provides a variety of online advertising services, offers a retargeting service called RoundTrip, which lets small and midsize businesses create and manage their own campaigns. There's a free two-week trial.

Fetchback
This company's focus is centered strictly on retargeting, and it offers some advanced techniques, such as displaying ads for items a customer has recently abandoned in his or her online shopping cart.

Google
In March, the search giant began offering retargeting services -- dubbed "remarketing" -- to its AdWords customers. The retargeted ads show up when users visit sites in Google's ad network.