A simple firefox extension that scans the URL for certain words or site names, and redirects to a blocked page if it detects such words.
The list of blocked words and sites are viewable in the options page, where you can also easily add or remove words from these lists.
This is pretty poorly designed and written, it was my done when I was much newer to programming. But I did publish it and a few people did use it, so it was a worthwhile experience