No problem! Nathaniel Kelso has already done all the work for us and we can download the extended OpenLayers plugin from his Github page. (There is a pull request in place to add Stamen to the default plugin too.)
Simply unpack the plugin into QGIS’ plugin folder. As you can see in the following screen shot, I’ve put it besides the “original” OpenLayers 0.6.2 plugin. In QGIS plugin manager, I then switched to Nathaniel’s version.
It adds the possibility to load Stamen’s Watercolor, Toner and Terrain tiles into the QGIS project:
Like all other OpenLayers layers, these can now be overlaid with more data such as this heatmap which is based on Twitter data I presented in a previous post, processed using Heatmap plugin and semi-transparently overlaid onto Stamen’s Toner map: