The second you log onto the internet, you start leaving a trace that's more telling than you think. Browsers can not only identify where you are in the world, but they collect a ton of other data too, such as where your mouse is hovering and when you launch a private browser window. Here's a way to find out exactly what you're leaking.
Point your browser towards this website experiment called "Click" to get started. A cascade of information will begin to stream down on the page, from the number of cores your computer has to the movements your mouse makes.
For the full effect, turn your sound up -- you'll get scolded for leaving the window, for example -- but be warned that there is swearing. The site is also able to compare your activity with the activity of other users and even guess information such as your gender from the way you're using the site.
Click is a rather playful web toy (check out the Achievements page if you want to see them all) but there are more serious tools around if you want a deeper and more formal dive into all of the data that's being picked up while you browse the internet.
Webkay also displays a comprehensive list of everything websites can learn and guess about you, without asking for explicit permission, based on the data reported by your browser. Not only does it explain what's collected, it also links to tools you can use to prevent some of the leaks from happening.
In the case of a lot of the entries you can install a NoScript browser extension (like NoScript Security Suite for Firefox or ScriptSafe for Chrome), which are able to block a lot of standard website sniffing code from being executed.
[h/t The Next Web]