Google has launched a new ad product that will allow web sites you've visited to keep serving you ads long after you've gone on to other sites. Which means that once you've visited viagra.com, you can never really leave.

It's known as "remarketing"—targeting ads throughout your web surfing based on sites you've previously visited. Google explains it this way, according to All Things Digital's Peter Kafka:

Let's say you're a basketball team with tickets that you want to sell. You can put a piece of code on your tickets page on your website, which will let you later show relevant ticket ads (such as last minute discounts) to everyone who visits that page, as they subsequently browse sites in the Google Content Network.

Replace "basketball team with tickets to sell" with "company selling a product or service that its customers might be acutely embarrassed about patronizing if their wives, or the people looking over their shoulders at a coffee shop, were to learn about it," and you get the idea. The ad banners on every site you visit could become a highly visible, blinking archive of every corner of the internet you've stepped in, whether you'd like to be reminded of it or not.

Google already has a similar program, called "interest-based advertising," that serves targeted ads based on Google's assessment of your browsing history. So if Google decides that you browse like someone who's into gardening, then it serves you ads from people trying to sell gardening tools. But the remarketing roll-out drills down from categories of interest to actual web sites you've been to.

Kafka thinks Google's rush toward highly specific, individualized ad targeting may wind up putting the company at odds with Congress, where some legislators are making noise about regulating the practice. If you want to opt out, and deny Google the capability to hijack your browser with ads telling you, and anyone who uses your computer or looks over your shoulder, what you've been doing on the internet, go here.