Henning Karlsen has a nice set of libraries for Arduino and ChipKIT. Recently he has updated his tinyFAT library that supports FAT16 formatted SD cards. Cards must be interfaced to Arduino SPI pins.
He’s been developing library for over a year so its polished enough to do serious projects. Latest release has lots of bug-fixes, careful rewrite of low level functions and of course it now has Arduino 1.0 support. The documentation of every function is very intuitive and well organized. Be sure to check out other libraries including touchscreen graphical displays, DS1302 RTC and many more.