Best site I have found is - europeportreviews.com - they advise on the best way of getting into town or around the area from the port. If you want to see a specific site which is some distance away it is probably better to go on an organised tour. If you just want to wander round generally most places are very easy to get to either by train or on foot at half the cost. Many cruise passengers do their own thing - just follow the crowd to the station.
Two more sites - cruisereviews.com and portreviews.com.
Yes, there is - google 'ports of call cruise.