K9 is available for Macintosh as well.
Opendns is also available for any computer for free. Just set (or have your local techie set) your dns server on your router to 208.067.222.123. This blocks the worst of the worst, although you may want something more comprehensive. If so, have your local techie set you up with the full opendns (customizable categories, and the techie will install the “dynamic dns updater” software for you, which is available on mac as well.)