MicroSD Shield and SD Breakout Hookup Guide
SD Card Breakout Boards
If you have a smaller Arduino (or you'd like to put a full-size SD card in your project), you can use the SparkFun microSD Transflash Breakout or the SparkFun SD/MMC Card Breakout with the SD card library.
Since these smaller breakouts don't have built-in level shifting, make sure you have a logic level shifter like the SparkFun Logic Level Converter in the circuit, or use a 3.3V Arduino like the SparkFun Pro Mini 3.3V/8Mhz
Both examples use Pin 8 as the chip select line, just like the shield example above.