8-channel Bi-directional Logic Level Converter - TXB0108 (ID: 395)
£6.62 ex VAT
Because the Arduino (and Basic Stamp) are 5V devices, and most modern sensors, displays, flash cards and modes are 3.3V-only, many makers find that they need to perform level shifting/conversion to protect the 3.3V device from 5V.
This chip performs bidirectional level shifting from pretty much any voltage to any voltage and will auto-detect the direction. Only thing that doesn't work well with this chip is i2c (because it uses strong pullups which confuse auto-direction sensor). If you need to use pullups, you can but they should be at least 50K ohm - the ones internal to AVRs/Arduino are about 100K ohm. 0.1uF caps have also been added onto both sides and a 10K pull-up resistor on the output enable pin so you can use it right out of the box.