Check around @ your lbs. In just about every metro-urban area is a group of people doing 'urban assault'. Riding after 10 PM through parks, alleys, etc. When I lived in Pittsburgh the group was mostly bike messengers and lbs mechanics. Similar to what wethepeople posted, except they were all on mtbs. Very subversive and pure fun. Be ready though. I went out w/them a few times and the ride can get pretty intense. But, you'll see the urban landscape from a whole different perspective.