Is there any way to get list of the cities that are displayed on a view port? If it’s possible, can I filter cities?
You can find some hints for that task if you go to http://help.openstreetmap.org and do a search there for key words like “list cities” or similar.
One option is to use Overpass API (http://wiki.openstreetmap.org/wiki/Overpass_API). It should be quite straightforward to build a query that returns all cities in a bounding box. You need to decide which OSM tags to filter for (i.e. what you consider a city), of course.