A variable DC power supply is the most useful tool on a hobbyist's workbench, because different electronics projects sometime require different DC supply. National semiconductor's LM350 IC can provide a variable DC voltage ranging from 1.25 to 33V with the use of just two external resistors. It can provide current up to 3.0A if properly heat-sinked.
This project uses a LM350 IC to generate a variable DC voltage from 1.25-18V. My finished product looks like this.