Don't quote me on the exact syntax (I do know the syntax for iptables):

ipfw add allow tcp from site-a.com to any http; ipfw add allow tcp from site-b.com to any http; ipfw add allow tcp from site-c.com to any http; ipfw add deny ip from any to any; sleep $HOWEVERLONG; ipfw flush;

I guess the other way would be simply to do ifconfig up when you need a site and take it down again. I don't tend to use such methods because if I'm really susceptible to distraction something else will distract me anyway. I think that self-discipline could be better (when the urge comes to load twitter, just don't do it).