Raspberry Pi is 3.3V based system. This means that it works with 3.3V powered peripherals. For fast hacking it actually can accept 5V signals without damage, but for more secure designs it needs proper level converter. Stuff4Pi made pretty good level converter board for Raspberry Pi which is bidirectional and covers all GPIO pins. Instead of using fancy chips to do this, he found an interesting NXP application note, where it is explained how using single MOSFET and couple resistor it is easy to make bidirectional level converter from 3.3V to 5V. Generally speaking converter works at different voltages like 1.8V, 2.2V, 2.8V as well. So it may be used as general purpose converter.
But main purpose was to attach to Raspberry Pi, because the PCB has a form factor that fitns nicely on Pi board. It also has a secondary header (which now is 5V) at same location, so it could be further used for other prototyping. Level shifter also has a dedicated LED for each line for monitoring status of each GPIO line. LEDs are buffered to avoid influence to normal pin operation. The one thing I am missing here is external power capability. We all know that Raspberry Pi power supply is pretty weak to run more serious stuff. Since it takes 5V from RasPi board we cannot escape the limited source problem. Probably additional jumper with optional external 5V source would be a killer.