Also, example.com/~username style URLs usually are 1990s-style static web sites where the only Javascript is something like a marquee. They are not the kinds of sites which have extensive Javascript or a web forum people can sign in to; at most a site like that would have a guestbook.
Are you kidding? It's a standard feature of Apache, used by universities all over the place. Many shared hosting providers offered the feature for customers that weren't ready to update their DNS to the new host, until quite recently.
