AmIUnique.org is a tool designed to show how users can be tracked across the Internet by their unique browser fingerprint.
Device fingerprinting or browser fingerprinting is the systematic collection of information about a remote device, for identification purposes. Client-side scripting languages allow the development of procedures to collect very rich fingerprints: browser and operating system type and version, screen resolution, architecture type, lists of fonts, plugins, microphone, camera, etc.
The site will show you exactly how it fingerprinted you, and whether you are unique among the previous visitors to the site (hint: you almost certainly are). The irony is that in many cases, plugins or settings designed to protect users from security threats can also make it easier to fingerprint them.
For example, the site tells me that only 31.39 percent of users visiting the site use a plugin designed to force https connections where available.