Safe Eyes (now owned by McAffee) and OpenDNS, along with a good firewall/rules on the computer. Safe Eyes only runs I think on OSX and Windows, so if you're a Linux man, you're out of luck here.
I've been happy with Safe Eyes as I've used it at the orphanage I volunteer at to reduce bandwidth (by blocking heavy video/music sites as well as to help monitor computers used by our older residents). They're pretty family friendly and they've donated the last couple years of subscription for us.
All in all, personally teaching and investing time in parenting and supervising what is done on the computer is a must. Placing computers in a high traffic area in the house and giving restrictions on when to use it aren't bad ideas either.