How can I change the settings on the vector "add feature" tool so the tool fill doesn't block my view of what I want to digitize? This problem popped up after I upgraded to QGIS 2.0
QGIS 2.0 Macbook Pro OS 10.9
Under Settings > Options > Digitizing, Find the "Rubberband" setting, Then "Line Colour" and adjust the "Alpha Channel" on the colour chooser.
I find that an Alpha value of 50 gives a nice level of transparency but of course you can tweak it as you desire:
Go to Settings -> Options. In the Options menu go to the Digitizing tab, then adjust your rubberband color. Drop your alpha channel value down, by default it is 200, the lower it is set the more transparent your digitized polygon will be as you build it. For me 50 was a good setting. If you set it too low you won't see anything at all. When I was adjusting it I created a temporary shapefile and experimented with different values until I came up with one that let me see my background photography but was still visible enough to be useful.
for QGIS version 2.10.1 it is: Under Settings > Options > Digitizing > "Rubberband" > "Line Colour" > Choose Colour > Opacity.
There u can choose around 50-70% to have desired results