Simplest uni-directional level shifter (3.3V -> 5V)
Maybe not lowest component count, but chances are that you have all parts readily available. Any general purpose low power transistor will do and resistor values aren't too critical either.

Possible disadvantage is that it acts like an open collector with R2 being the pull up resistor. This means the rising edge is determined by R2. The 3V3 output driver must be able to sink the current through both R1 and R2, in this case (5V/6k8)+((3.3V-0.6V)/2k2) = 2mA.







simulate this circuit &ndash; Schematic created using CircuitLab

Not sure where I once found the circuit.

Similar architecture can be used with a MOSFET, but you'll have a challenge finding one with sufficiently low threshold voltage.