Monitor employee internet usage

Tzvi  writes...

Hi guys,

First, I love the show,keep up the great work.
I also hope you get a nice kickback from Newegg, because at my work place I started doing all our purchases using your referral link(I'm a network admin..).

Second, my question is, can you recommend something to monitor employee internet usage, company has about 50 employees. I prefer open source or free, but anything will do.....

Thanks guys!