For my kids I use We-blocker:

http://weblocker.fameleads.com/

Itis completely free and fully functional at that price.

It has fully functional filters and can block by any URL you add to the block list. It can even update automatically from the list server. You can control the content with built in category filters.It will also keep a record of sites visited if you set it, but since you are talking about an office environment, you better check your legal for that aspect.

Here is the disclaimer, it only works with MS IE, not mozilla or firefox. And it is fairly easy to defeat if you have a little bit of a clue. But I keep an eye on it and keep both of my tenagers at bay, (although I have other things going on as well and no legal issues to deal with....yet)

Hope this helps.