ProCon Latte Content Filter is a lightweight browser extension which can filter access to pornography or other material.
We installed the package, tried searching for "porn", and were immediately told "this page is unavailable due to policy restrictions".
If a site is blocked incorrectly, you're able to allow access temporarily, or add it to a whitelist to ensure it won't be blocked again. (All these options are password-protected to prevent others changing your settings.)
The filter is based on local keyword lists, and isn't as accurate as some of the more sophisticated competition. But it's also very lightweight, and doesn't compromise your privacy by sending URLs or other browsing details to any central server.
Most of this can be disabled or tweaked in the Settings dialog, and there are some handy bonus features. You could disable a custom warning message, for instance; redirect blocked sites to another URL, or even restrict browsing to a custom whitelist of preferred sites.
ProCon Latte Content Filter is easy to set up and use, and enables blocking a lot of adult content in just a few seconds. The protection is also easily bypassed, like all browser-based filters, but there's still enough power here to keep young children away from the worst of the web.