This demo demonstrate the use of the GeoExtUX_ZoomTo widget, which consist in a popup that allow the user to recenters the map on a user defined coordinates.
You can either use it without any projection list or specify your own. In the former case, EPSG:4326 and EPSG:900913 are used. In the latter, each projection defined automatically loads the according proj4js file containing its definition as demonstrated in this demo which uses EPSG:42304 and EPSG:4326 as defined projections.
You can drag the marker around using the "enableDrag" widget option as demonstrated.
When you pick a different projection, if the X and Y fields contain coordinates, they are automatically reprojected in the newly selected projection.