I believe Netcraft still produce this sort of report on a monthly basis and use it to record (among other things) the relative popularity of webservers. At leas

I believe Netcraft still produce this sort of report on a monthly basis and use it to record (among other things) the relative popularity of webservers. At least, they did when I worked there many moons ago; and they monetised such technology. Biggest hurdle isn't the software or execution time: rather, it's making an agreement with your ISP/hosting company wherein they allow you to portscan the entire internet on a regular basis without flagging it as abuse.