I'm using the QGIS google layers plugin to digitize a parcel as a shapefile. Later I want to import it to PostGIS. I know that google maps use a special 'google mercator' projection. What is the correct code of this projection: 900913 or 3857? Or are these two codes describing the identical projection equations?
900913 = GOOGLE (spelled with numbers). There is no official EPSG code 900913, because for some time this reference system wasn't added into the EPSG list of spatial reference systems. After some time, it eventually was under the code 3857 (and there was even some other code for some time). Looking at their properties, they are the same, so you could use one or the other. Doesn't matter
EPSG: 3857 is the official one you should use though, because other names are considered deprecated: http://wiki.openstreetmap.org/wiki/EPSG:3857
I have always used EPSG code 3857 for the web mercator, which is backed up by the always useful spatialreference.org.
Incidentally, I could not find 900913 on spatialreference.org, but when I searched for it, one of the results it did come up with was 3857.