Last time I opened QGIS there was a notification that there was a new plugin; the auto-trace plugin. I just tried it out to see what it is all about. I must say, it is a pretty nifty tool. It let’s you digitize new features based on other features by tracing existing features. See here for a short explanation how the plugin works, including an instructional video. It is not a thing I would use often, I don’t do too much digitizing, but I can see how this can be very handy.
If you want to try it out, it is still experimental, so you need to enable the experimental plugins in the plugin settings.
You have come this far, so I assume you are interested in digitizing using QGIS. So let me give you an little bonus tip: check out this blog post about topological editing in QGIS.