What hardware information about your PC's can gathered by websites/ISP's?

Using a mixture of flash, java and javascript, a website can find out:

• What browser + the OS you are using
• What fonts are installed
• Your screen resolution and colour depth
• What plugins are installed
• Approximately where you are

Hardware can only leak out if there is a specific plugin for it (not a usual case), or if some activeX checks.

A new HTML5 element device will allow use of webcams etc, this will likely be an abstraction though and will only say whether you have one or not, rather than the brand.

Have a look at https://panopticlick.eff.org/ for an interesting look at what your browser tells a site.

As far as I know, your MAC address isn't exposed to any website - if it was, why would, say, advertisers bother with trying to track you via so many different methods, when there was one foolproof one? None of your PC's hardware specs are included in the request, though your browser will be exposed in its User Agent string, and that often includes OS, too. You can change that trivially, though, so it's not foolproof by any means. Your ISP can't see your mac address either, iirc, as I do not believe that ever leaves your local network, assuming you're connecting via a router.