Some websites, like facebook, youtube, are rather time consumable.
If you do nothing to filter certain websites, your employees may spend several hours a day on web surfing.
So How to block certain websites to save your productivity?
1. Some router/gateway might have the ability to block certain websites.
2. Firewall appliances, like cisco PIX, will also be a good choice.
3. The third, you can choose internet filtering software to do web filter and blocking.