Adafruit 4-channel I2C-safe Bi-directional Logic Level Converter

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 level shifter board combines the ease-of-use of the bi-directional TXB0108 with an I2C-compatible FET design following NXP's app note.
This breakout has 4 BSS138 FETs with 10K pullups. It works down to 1.8V on the low side, and up to 10V on the high side. The 10K's do make the interface a little more sluggish than using a TXB0108 or 74LVC245 so we suggest checking those out if you need high-speed transfer.
While Adafruit designed it for use with I2C, this works as well for TTL Serial, slow <2MHz SPI, and any other digital interface both uni-directional and bidirectional. Comes with a fully assembled, and tested PCB with 4 full bidirectional converter lines as well as 2 pieces of 6-pin header you can solder on to plug into a breadboard or perfboard.
TECHNICAL DETAILS

BSS138 Datasheet
EagleCAD PCB files on GitHub
Fritzing library

The newer NXP app note explaining how it works (the older Philips version of the app note is also worth a read)