I was thinking on tools/plugins for QGIS, but other good suggestions are welcome.
The OpenStreetMap (OSM) editor JOSM is a great tool for digitising. Although it is specifically developed for editing data in OSM it can save data as .osm or GeoJSON files which can then be converted to other GIS formats (e.g. using ogr2ogr). This export facility converts it into a good general purpose digitising package.