Controlling access or fool proof monitoring of employees internet use.

I run the maintenance crew for a hotel here in San Diego and after several years of requests I've gotten a computer for the shop so my Engineers can do things like research part information and program things but I want to weigh to be able to monitor their internet access and limit it to only the sites they should be accessing. I know there's pricey kiosk software you can get to do this but what are the relatively cheap or free options to do this? Are there any browsers that would accomplish this by say blocking all websites accept specific ones that I pick? What other ways can I accomplish this?

I look forward to your replies and will appreciate any help provided.

Mark in San Diego