What is answering at port 80 when httpd is down? Since OS X Lion, web sharing isn't available from the System preferences. 

So I shut it down using sudo apachectl stop. Doing this, http://localhost/~Username doesn't answer any more and all httpd are down (a least ps -e | grep httpd return none of them).  

But using my browser http://localhost still returns a "It works!". Who then is answering at port 80 ?
Nothing. Your browser is likely returning a cached response. Try Shift+Reload or clear your cache and reload. Then, you should see no response.