That depends on what you mean.
Your MAC address is specific to your networking hardware-- i.e., your wired and your wireless connections each have their own MAC address. For TCP/UDP, this address doesn't get propagated past your router.
Some applications, however, may use it when submitting reports or for authentication mechanisms-- in these cases, they would have to read the MAC address directly and then submit it to something. For instance, an application may generate a hash of your MAC address and other identifiers on your system to use as a registration key that would only work on your computer. Obviously, for such an application to get your MAC address, it would have to be running on your machine-- it won't get sent out if you're just reading e-mail or browsing the web.