K9 web protection works very well for me, and it is free. It is also very configurable, and if it blocks a page that you didn’t want blocked, you can override with a password. Just make sure not to lose the password, because you need it if you want to uninstall or make any configuration changes.
You could also try OpenDNS, which works through a setting on your router, so that it filters your entire home network, not just one computer. It is also free.
However, no filtering software is perfect. It is especially very difficult for software to block objectionable content on sites where users can upload their own content, such as YouTube or photo sharing sites. It is much easier for software to block entire sites, such as porn sites. (But even then, it could possibly miss some sites.)