Whitelist
Most Internet filters works with a blacklist, which means that access to all sites is allowed, except some special sites (well, sometimes there are a lot exceptions...). A whitelist works the other way around: Access to all sites is blocked, except some special ones. For a normal ISP it is almost impossible to offer, because the Internet is nearly worthless. The whitelist scheme is used by free Internet terminals that are sponsored by a company which allows users the free access to their e-commerce site. This filter scheme is the most difficult to circumvent (if proper implemented impossible!).
Some time ago, there was a German ISP who had a completely free 0800-dial in number. Once you dialed in, you only could surf to amazon.de and about 10 more e-commerce sites. But you could also connect to the other customers of the ISP. So somebody with a flatrate connected to both his normal ISP and the 0800-free ISP and set up a proxy. So all the users of the free ISP could use that proxy to connect to other sites.