I made a temperature and humidity logger. Now I know there are probably thousands of loggers around, but this one was designed to have very low power consumption, which isn't really possible with a development board and a shield on top.
During idle mode (almost all of the time) it only uses around 6 µA to 7 µA of current. This means it should be possible to power it from batteries for a long time.
Readings are taken every 15 minutes and logged to a tab-delimited file on the micro-SD card. This can then be imported into a spreadsheet, eg.
Schematic and detailed design descriptions here: