While NetDog does block some harmful sites, its patchy implementation leaves a lot to be desired.
Basically, the app is a "smart" blacklist that comes with many sites and keywords preblocked and allows you to add or remove others as you see fit. There's also a white list of "always allowed" sites. Interestingly, it will allow some sites that talk about the human body in scientific terms, unless you block them explicitly. Unfortunately, the "smart" blacklist didn't block about one-quarter of the sites we tested that we thought should be blocked, making the usefulness of the feature dubious at best. The "full" blocker functioned much better, allowing nothing of a questionable nature to show up.
The interface is a one-panel job, with all the options front-and-center and password-protected. Since the app is so simple, it would benefit from a less-compact menu, but what's there does the job decently enough. If all you need is a Web site blocker, this free application is it, but you can certainly find these limited features and a lot more in other parental-control utilities.