"ArcGIS REST API Connector" (short name: ArcGISConnector).
This plugin is in alpha release phase (QGIS experimental). Beta release (still QGIS experimental) is planned as of end of august 2015.
A QGIS-Plugin which reads "Feature Services" - aka vector-based, WFS-alike services - from ArcGIS Online or ArcGIS Server. ArcGIS REST API includes also "Map Services" - aka raster-based WMS/WMTS-alike services - are currently NOT supported (and won't be supported in near future).