(FORTUNE Magazine) – Almost everywhere you go on the Web, you are tracked by tiny software programs called cookies. If you get quotes on your favorite stocks from AltaVista's Website, the ticker symbols you enter are spirited to Internet ad agency DoubleClick. Search for information on Alzheimer's disease on Yahoo's health pages, and AvenueA, another advertising and marketing network, finds out about it. Want to ask ask.com a question? AdKnowledge will hear it too.

Sure, cookies help sites provide you with personalized service--for instance, book recommendations based on your past purchases. But data about where you go and what you do is also sent to advertising consortiums, which assign your browser a number and follow you wherever you click. While most sites have privacy policies that prohibit sending such information as your name, phone number, or e-mail address to these third parties, chances are that if you surf the Web long enough, marketing firms will be able to construct a detailed picture of who you are.

There isn't much Web users can do about it--most programs that block cookies also interfere with your ability to surf the Web. But now an Israeli company named IDcide (pronounced "I decide") is offering a program, called Privacy Companion, that is able to distinguish between local cookies (the kind sites use to give you better service) and third-party cookies (which marketing firms use to track you).

Available free at www.idcide.com, the software lets you block unwanted third-party snooping. The program, which is simple to download and install (but isn't yet available to Macintosh users), puts a series of small icons in the corner of your browser screen. The icons indicate when you're being tracked and specify whether it's a local or third-party cookie (assuming you haven't selected the option that blocks the latter). Clicking on these icons brings up detailed information about which sites or companies are following you.

So far, 50,000 users have downloaded the free software since its March launch. But the eight-month-old company isn't likely to be profitable anytime soon. It hopes eventually to make money by creating a program that will allow consumers, who are wearying of cookies, to carefully control the information they give out. Theoretically, Websites--wanting to keep their customers--would then pay IDcide to make their sites accessible to this software. Till that happy day arrives, IDcide helps shield users from one of the annoyances of the Web.

--Jeremy Kahn