Outside of spam, dangerous websites with known trojan, and maybe obvious porn. Why would you want to block your employees? I've worked once for a big company like this. I left. A lot of websites were blocked. Even craigslist. Led to workarounds and other hacks. It was also quite counter-productive in many ways.
Honestly if you don't trust your employees don't hire them. If you have employees that aren't productive because they are doing things they shouldn't be doing then let them go.
I wouldn't work for you.