A MAC address is unique only to a network card, and not the computer itself. Apart from that MAC addresses can be easily changed, and as stated there is no method to get that information from a web browser.
Apart from your normal cookie tracking, you can record a variety of parameters (IP, Browser Version, etc), which can give a decent result for tracking over time. There isn't any 100% way to keep track of users through, and even a complex parameter tracking system is subject to flaws, because the user can change things on their browser, which nullifies your previous record of them. Most people settle with session or cookie tracking, and just accept the fact that you cant keep track of everyone.