Here’s a simple adjustable current source with a PNP transistor that drives either one or two LEDs for testing and/or intensity matching purposes. The current is from 2 to 16 mA in four steps using a rotary switch. While this would make a good beginner’s project, its also a useful gadget for hobbyists of all skill levels. The project uses a handful of resistors, along with several transistors, diodes and switches. Simple build instructions can be found in this three page PDF.
Via Jameco.