How can I determine what hosting provider hosts a site?

Is there a service that will identify where a site is hosted (presumably by IP)?

A good place to start is whois:

ARIN IP Address whois lookup: this allows you to see who "owns" (ie. who is the registered operator for) a given IP address block.

Internic whois lookup: this looks up the registered operator of a given domain name, along with their DNS servers (which may be operated by their hosting provider, but which could also easily be hosted elsewhere).

That will get you started. For more detailed information, use the traceroute command (or "tracert" on Windows) to see the path between your point on the network, and theirs.

Can't believe no one mentioned Netcraft

"What's that site running?" Top left hand corner.