When www goes missing

Remember when we used to type "http://www." before every single URL that we typed in? And then people learned to leave off the whole "http://" mess? I started leaving off the "www." quite a while ago, and for the most part it works just fine. Most websites are set up to accept www.utilware.com or utilware.com, and both URLs point to the same thing. However, if you make a habit of always leaving it off, it's surprising how many sites end up serving you a 404 or other strange response instead of their homepage.