Proxy caches are often used to prevent users from accessing certain Web sites or pages. This activity is also known as blocking and censoring. The ethics of such filtering is the subject of an ongoing debate, but it is not discussed here. Rather, here are some pointers to other Web pages with relevant information.
PeaceFire is an excellent resource. They provide a lot of good information about they way that particular products work -- or, don't work.
You might try your luck with the censorware.net site.
Some Internet users may be able to bypass local filtering proxies by utilizing a third-party proxy elsewhere on the Internet. Anonymizer, ProxyMate, and BeSilent are all services designed to protect users privacy. Some require a subscription. The proxys4all Web site has some information about open-access proxy servers, as does the Anonymous Internet Surfing HOWTO.
W3C's Platform for Internet Content Selection (PICS).
Rating the Net: Jonathan Weinberg. in Hasting Communications and Entertainment Law Journal, Vol. 19, No. 2, p. 453-482.
N2H2 found itself in trouble for selling its ``Class Clicks'' lists to marketing research firms. EPIC made a FOIA request to the U.S. DOD relating to N2H2. Commercial Alert has a nice list of articles about N2H2.
DANGEROUS ACCESS, 2000 EDITION: Uncovering Internet Pornography in America's Libraries. This is a lengthy booklet produced by the Family Research Council.