Bench PSU - Schematic
Resistors R1, R2, R3, R4 are there to draw enough current to get a stable voltage from the regulators (the datasheet only guarantees the voltage regulation within a certain current range, and I have found that especially with the 79xx series the voltage can be way off with no load). The other 3 regulators 7815, 7805, 7915 do not have these resistors because there are resistors and LEDs across the outputs (not shown on the diagram) that draw sufficient current.
Improvements that could be made:
Include additional 1N4004 diodes between the output and ground of each regulator, to protect against reverse voltage applied to the outputs.