Recently I ordered some breadboards on ebay. The order included 2 YwRobot 545043 Breadkboard Friendly power supplies.
YwRobot 545043 Powersupply Inserted into 2 ¼ inch wide breadboard
The power supply fits into a 2 ¼ wide breadboard (2 rails). You can power each rail with 5V or 3.3V separately or turn off either rail. With the proliferation of 5V and 3.3V microcontroller boards (Arduinos and mbed devices like the mBuino) having an inexpensive and versatile 5V and 3.3V power supply handy is important.
Just set the jumpers for each rail supply voltage you want and insert it into one end of the breadboard.
I hope you find this useful, if you do please leave a comment.