Policy.com is nice site and is designed specifically for political discussions. Their
roundtable is well organized. You can chose between
Education,
Family,
Health,
Society,
Environment,
Politics,
Economy,
World and then each one of these has
sub categories. What makes it so good though is that it also includes a whole bunch of
resources on the different topics that you may wish to discuss. Its not super monitored and the discussions are interesting and because of the resources and level of understanding that each person (I mean if you go to a site called policy.com presumably you understand political science) brings to the discussion are very
enlightening.
Another good place to look is at the main portals, like yahoo, news section where there is usally a forum about current issues.