[REQ]Visitor Demographics Tool for Other Websites

I dont know if this question would be allowed by the moderators, but if i want to sneak in and find the visitor demographics for web sites not owned by me . how do i go about it ? Besides the traffic wouldn't be all that awesome ? are there any online tools other than alexa and such ? the traffic doesnt justify such websites to be linked on alexa ..just a noob question. been googling and didnt find much help