Beyond IP tracking, the EFF's Panopticlick website demonstrates how much uniquely identifying is exposed from a browser's User-Agent and system configuration values accessible from JavaScript and Flash (such as screen size, locale, and installed fonts). For example, my browser's fingerprint is unique among the 1.7M browsers the EFF has tested to date.

https://panopticlick.eff.org/index.php?action=log&js=yes