Privacy.Net Analyzer is one of many tools on the web that will inform you of what information your browser is leaking to websites. It is one of the cleanest and best looking of these tools that I’ve seen, however.
It will attempt to determine information about the computer you are using, potentially vulnerable autofill settings, other websites your browser is logged into, and a fingerprint analysis.
One of the things I was happy to see was how little information the site was able to determine about my browser, thanks to actions I’ve taken to limit that.