Shodan is currently giving their lifetime memberships for $4 US as part of celebrating reaching 4million users. I am unsure how long it will last.
Good deal as they skipped their usual cheap membership deal on thanksgiving last year.
What it is, from Wikipedia:
Shodan is a search engine that lets the user find specific types of computers (webcams, routers, servers, etc.) connected to the internet using a variety of filters. Some have also described it as a search engine of service banners, which are metadata that the server sends back to the client.[1] This can be information about the server software, what options the service supports, a welcome message or anything else that the client can find out before interacting with the server.
Something, something, Polito form.