I got a quick question! Is the circuit depicted in the link indeed a voltage doubler or is it just a bridge rectifier with smoothing capacitors?
It's a standard full BR with smoothing capacitors (with the option of using the center tap as circuit ground and having positive/negative rails)
One type of voltage doubler that looks a little similar is this:
Note that the above is assuming nothing about the transformer winding ratio, only whether the secondary voltage is doubled by the following circuitry.
The circuit pictured is a fancy BR circuit, because of the center tap connection, I would suspect that is only slightly better than just seperating the output lines with a capacitor, the more typical application with a center tap is to have a common ground with both + and - voltages out of the rectifier.
also with center tap, you can make a full wave rectifier with just 2 diodes (rather than the normal 4)
The reason that it isn't a doubler is that there aren't 2 times as many coils on the load side of the transformer (if the drawing is to scale.)
It is also possible with multiple taps to have multiple potentials eg., -15 -7, 0, +7, +15 (ish unregulated, VAC).