which is best Free webfilter software.?

hi,
is any free web-filter/proxy server software? i need to filter all the websites like social networking sites, video sites, game sites,etc…..
all users are working in online, but some of checking their mails, watching videos at their working time. i want to restrict them. is any free software is available for monitoring and web-filtering?
I’m using 2008 server, all the users are domain users….
thanks in advance……..
sorry for my bad English.