Website blocking is much better done at the router/firewall level, since it is much harder to bypass.
There's not much free stuff out there for enterprises. Most software web filtering is designed for parental control and home use since enterprises tend to use UTMs or hardware firewalls.
Pfsense (
pfSense(R) - World's Most Trusted Open Source Firewall) and Untangle (
Untangle: Network Policy at Work) are open source router operating systems that you can install on relatively low end hardware.
If you already have a firewall solution, chances are that will be able to help. Modern NGFWs from Cisco, Juniper, Sophos etc can all do web filtering. Even older firewalls can do basic web filtering and blocks.
Some Anti-Virus solutions will also come with a web filter, so if you have an endpoint AV solution, it's worth seeing if it offers something you can use.