hi, the only one that i can say is the best and free is. Blue Coats K9 web Protection click here
this is one of the best and is free. not only does it just block catagories, ie pornography and email. it also enables you o block full web address such as miniclip.com
It also enables you to monitor all of the activity that is done on the internet this will log down all of the websites visited all that were allowed and denied. You can then evaluate allowed websites and decide for yourseld wheather the website was appropiate.
Another great feature is the ability to block keywords. so if you found you children going on websites that are not being blocked ie if they type in the word online games, and if you wished them not to go on them. Then the software will is able to blcoked any web address and any search conducted containing those certain words that you set. So if you blocked the word miniclip, if i went to miniclip.com i would be denied and it i typed miniclip in any search engine ie google then i would be denied from seeing the results. K9 web Protection is the best parental control software that i have ever used and is free. Donyt worry your children will not be able to disableit as you need a password to overide the control to gain access to a website and the same password to edit or uninstall the software configuration
Give it a go and download it today. Registration is required and you will recive a free activation code by email.