WHEELES  I've been using this for a while now and have experienced no problems with crashes or odd behaviour. The features of this software far surpass many competitors.

This free software enables the administrator on the machine to select which categories should be blocked and also allows time limits for Internet usage to be enforced.

While the software does refer to an extensive blocklist, the software makes an intelligent guess as to the nature of those sites not already categorised. The software offers the ability to submit sites for review that you consider are incorrectly labelled or not labelled at all. I have submitted quite a few sites and the turnaround from submission to judgement and re-categorisation has only been a few hours. You can also add sites to your own personal blocklist if desired.

I've tried several content filters in the past and have also used OpenDNS to enhance my content filtering. This is by far the best one I have used, and given that it is FREE makes this one head and shoulders above the rest.  
(Version 0.4)

SANS DIEU RIEN  As a concerned parent of two, coupled with the escalation of dangerous web-surfing, I'm always keeping an eye out for practical ways of protecting my kids without stifling their growth. Yet, I'm a firm believer that no program is perfect, and is a poor substitute for actual parenting.

K9 takes the common route of many internet protection programs by creating an extensive blacklist, but adds an extensive vigilant response from customers or parents with an extensive category system of various levels. I can feel my self surfing and wondering if I should report a certain site, or tell them that the site is just mislabeled. Perhaps many would just ignore it. In my 30 mins. of surfing, I reported two sites I thought were definitely in the wrong categories.

This is probably why the program is free. It takes diligence on the parents' part. At first, I thought that it would be better if K9 just did it accurately and automatically; but then again, as I thought more about it, it indeed all comes down to us, doesn't it?  
(Version 0.4)

TAS7BLUE  I like the idea of K9 Web Protection, but every time I did something on my MacBook after having installed K9, my computer would randomly and without warning force me to shut down. Also, I couldn't get the K9 icon to show up on my Firefox web browser.  
(Version 0.4)