AltaVista Family Filter
Enable it by selecting "AV Family Filter" from the links below the main search box,
in the "Specialty Searches" section.

Goguardian
To enable, select the GOguardian link below the search box, on the home page.
Even if Guardian is not enabled, Go will warn if it thinks search results contain 
possibly objectionable material.

Google
Select Preferences to the right of the search box, on the home page.
Scroll down to Safe Search Filtering and select it.