We've developed a number of productivity tools to help users with tracing / trace digitising.
Following on from the AutoTrace plugin (see below) QGIS Trace Digitising brings faster and significantly more advanced tracing features to the core QGIS application. The functionality was merged with QGIS' core digitising tools in QGIS 2.14. Visit the QGIS Trace Digitising page for more information.
Users of QGIS 2.12 or earlier can still get tracing functionality by using the AutoTrace plugin (see below).
AutoTrace is a plugin for QGIS that allows users to digitise new features based on vertices from existing features. It essentially allows you to trace existing features.
Visit the AutoTrace plugin page for more information on how to install and use this plugin.