Post Map is a WordPress plugin by jaffamonkey that adds a location field to the Add/Edit post form. You can type a full address, or simply just a postcode/zipcode (best to add country also e.g. SW 87B, UK). To display a map, there is a “Post Map” widget, with option to set Zoom level, map width and map height..
Download: http://wordpress.org/plugins/post-map/