Content filtering programmes work reasonably well in most buisiness environments, as far as they go. I've implimented MIMEsweeper and Websense, and haven't had big problems. They're fairly simpleminded, and do give false positives. There are also ways to circumvent them, if you are persistent.
Please see http://www.aclu.org/issues/cyber/box.html for a good discussion of why they aren't a good idea for an ISP or a Library, for example.
You have a much bigger issue, however. I've been there. By and large, employee surfing is an HR issue, not a technical one. To stop porn surfing (or any other non-buisiness surfing), there are a few things that have to happen:
1. The IT department must be able to fairly unambiguously tie a person to a log entry. This can be difficult places where security is weak (group accounts, lax authentication proceedures, etc.). If someone has the room to say "It wasn't me", then you will have problems.
2. Upper management must decide that porn surfing, et. al. must stop. Get your legal department involved; porn crusing has led to a number of costly sexual harassment suits lately. I'm sure once legal does their homework, faster than you can say "due diligence", they will demand that management take action. Further, upper management must be willing to follow through on punishing violators.
3. Human resources must write up and publish (in the employee handbook, usually) a policy stating that porn surfing is unacceptable, and action will be taken against those who do it. Action should be dismissal, in most cases. HR must be willing to follow through on punishing violators, and upper management must back them up.
4. The IT department must announce that per HR and upper management direction, they are going to be blocking sites, and that there will be logging of violators. IT will have to allocate time and resources to deal with the logs, investigate and make the appropriate reports. HR must be willing to follow through on punishing violators, and upper management must back them up.
5. Violators must be punished. If people know that even if they get caught, nothing will happen, they will thumb their nose at you. If the guy in the office that everyone knows to go to for the latest porn site passwords gets fired for inappropriate use of computing resources, people *will* take notice.
- Ken
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