I’ve just released the new experimental plugin Geometry Exporter for QGIS. The plugin displays the geometry of a selected feature as text. Thanks to GDAL, the following formats are currently supported: WKT, EWKT, GML2, GML3, GeoJSON. Also on-the-fly conversion (e. g. Envelope, Centroid, ConvexHull, Boundary) and transformation (changing Coordinate Reference System) are supported.