The Profile plugin
From AWF-Wiki
The Profile tool plots terrain profiles along interactive pointed lines. It handles one-band rasters of any format supported by QGIS.
After installing and enabling the plugin, you will find the “Terrain profile” button in the plugin toolbar: 
Basic use case
- Load the elevation raster and select this raster layer in the layer list.
- Press the “Terrain profile” button. This opens the plugin panel which consists of a graph area on the left and a raster layer list on the right. The raster layer you had selected will be added to the raster layer list. If you have “Selection: Temporary polyline” enabled, you can now draw a line in the map area.
- Double-click left to end drawing the line.
- After you have finished drawing the line, the graph area will update and display the profile
