Extracting Email Addresses

I was wondering if anyone out there could help me. I have a disscussion with a friend of mine that it is possible to extract email addresses from people who visit certain websites. That means that for example I visit a website that sells plants, and then the following day I receive in my junk mail offers from different vendors selling me their plants. Is there a software that does this and if so would it only monitor people who visit my website or would it also be able to monitor and extract address from other websites, other than my own???