Looking Glass servers are computers on the Internet running one of a variety of publicly available Looking Glass software implementations. A Looking Glass server (or lg server) is accessed remotely for the purpose of viewing routing info. Essentially, the server acts as a limited, read-only portal to routers of whatever organization is running the lg server. Typically, publicly accessible looking glass servers are run by Internet service providers (ISPs) or NOCs.
One of the most complete all-in-one list of looking glass resources of different providers:
One more list of looking glass resources: