I'd recomend, like others, a router based solution.
I have not used a consumer router (linksys, etc) in quite some time, but I hear they have some options.
I know the free open source options for building your own (smoothwall, IPcop, PFsense) all have very very slick ways to block content and services (IM, IRC, etc) and thats the route I will take when its my turn to face the same thing...but admittedly, its complex to setup and administrate.
I have not used it, but Astaro touts a product that is best of breed open source and commercial software and their home version is free (bring your own hardware) or get a router-sized box for pretty darned cheap
http://www.astaro.com/our_products/a.../web_filtering