Floob said:
I was wondering what method you would recommend for measuring traffic on sites that you dont own (both for sites that are live and those that are just registered)
Well, you can ask the owner to put up a public web tracker for you to peek into, or send you the admin password for their server logs.
Anything else eg Alexa, is just a toy gun for the kids to play pretend.
Generally, if you want to know such stuff, you need to seek the consent of the owner. Peekaboo, even if technically possible without that, is invasion of privacy pure and simple.