There isn't a simple answer to this question. IP addresses change constantly and MAC addresses can be changed. Motherboards and other components often have a programmed identifier like a serial number, but I don't believe there is any kind of standard to access them from software, and it's certainly nothing a website would be able to access to identify you.
The best a website can do to identify you without your knowledge is through your IP address or by setting a cookie and tracking that. However, your IP can change (especially if you are a laptop hopping across different wi-fi networks), and you can clear your cookies. Flash provides some storage for Flash applets but you can clear this as well.