Dynamic site which can allow users to surf whatever other sites without leaving it

I need a dynamic web site which can allow users to surf whatever other sites without leaving it With this site, whenever its registered users surf other static or dynamic sites, they will be able to do so because those sites will be displayed within this web site I will built. If the other site is dynamic and requires log-in in restricted member-only area, this site will be able to allow its user to do so without having to leave it to navigate the other site independently. And there will be other dynamic features of this site, but I need an outline of your solution to the above-mentioned need first.