K9 Web Protection
One nice feature is that you can control what happens when someone attempts to access a Web site you have blocked. For example, you can play a sound (so that a parent will hear an alert if access is attempted), and there is even an option to completely shut down Internet access for a period of time if someone repeatedly tries to access blocked sites.
Before you can install the software, you have to request a free license key. The license request process can be initiated through the installation wizard.