Be a little careful on those. It depends on what you're doing. Some of them are not suited to be used with the high data rates for I2C, or I2C only at 100khz. I found out the hard way with some of the SparkFun level shifters, years back.

You need to do a little research. It will usually tell in the spec sheet. Which is why the Arduino is useful. You don't have to buy a level shifter. You don't have to read a level shifter spec sheet.