Some companies do that with http headers (e.g. Zappos, SEOmoz, GigaOM)
$ curl -v www.reddit.com
> GET / HTTP/1.1
> User-Agent: curl/7.30.0
> Host: www.reddit.com
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: text/html; charset=UTF-8
< Server: '; DROP TABLE servertypes; --