Here is a useful Arduino shield. Many people have been looking for a dedicated and well-designed data logging shield. Adafruit worked hard to engineer an inexpensive but well-rounded design. Not only is it easy to assemble and customize, it also comes with great documentation and libraries.
You can get going quickly - saving data to files on any FAT16 or FAT32 formatted SD card, to be read by any plotting, spreadsheet or analysis program. There is a tutorial on how to use two free software programs to plot your data. The included Real Time Clock timestamps all your data with the current time, so that you know precisely what happened when!
Please note that this item does not come with an Arduino (you'll need one to use with the shield), or an SD card. It does come with the RTC battery, however. The shield now comes with all the components soldered on and tested but does not have headers installed. You'll need some basic soldering skills to put it together, but even if you don't have much experience you can get it done in less than 15 minutes.
SD card interface works with FAT16 or FAT32 formatted cards. 3.3v level shifter circuitry prevents damage to your SD card
Real time clock (RTC) keeps the time going even when the Arduino is unplugged. The battery backup lasts for years
Included libraries and example code for both SD and RTC mean you can get going quickly
Prototyping area for soldering connectors, circuitry or sensors.
Onboard 3.3v regulator is both a reliable reference voltage and also reliably runs SD cards that require a lot of power to run