According to Symantec, more than 80% of attacks on the internet are aimed at a web browser or website. Hackers are using additional features of browser, such as ActiveX, Java, Cookies, etc to gain this.
Here I am not talking about how to prevent these attacks, instead, just aware you what your browser sends information about you to every website you visit.
This is useful; for example, if you are using a proxy server to hide your IP address, make sure that your IP address does not appear to others; if it does, you should change your proxy.
Following are some online tools, just visit any of the sites and see what your web browser show about you
BrowserSpy - Tell you all kinds of detailed information about you and your browser. Information ranging from simple stuff like the name and version of your browser to more detailed stuff like what kind of fonts you have installed and what hardware you're running on.
Audit My PC- Another similar service which promoting security awareness and web security tools free of charge. AuditMyPC is a vulnerability assessment, privacy test, and research and security information site.
HackerWacker – Another service which reveals what your browser is telling every website you visit.