You’d be surprised how many of your favorite web pages use web bugs.
What are web bugs? They are similar to cookies (except they are not placed on your computer, they are imbedded in a webpage); they are a 1x1 pixel picture (general white or the schema of the page) placed on a web site (there could be more then one) that will track where you came from or just about any other data from your computer. Here is FREE software that I have been using for about 2-3 months, might be worth a look.

www.bugnosis.org/