Quantity   Component name
1 × Any pro-mini style 3.3v Arduino ( $2-$15 ) i'm currently using Rocket ultra boards, but have working builds with many others including some fairly dubious no-name clones. Note that you must use a 3.3 volt Arduino to connect the SD card directly to the pins! Also note that the pin layouts vary from one board to the next so you will have move some jumpers to accommodate that.
1 × MicroSD card adapter for Raspberry Pi ( $1-3 ) I prefer the ones from Adafruit, but just about any one will do. In fact my first few loggers used the little uSD adapters that came free with the cards, but soldering those was tricky.
1 × DS3231 & 4K I2C eeprom RTC module ( $2 ) I prefer the DS3231 over the DS1307's because they have much less drift over time. But both work with the same libraries.
1 × μSD card - 128mB or larger ( $2-10 ) Fake SD cards have given me more problems than I care to mention. It is crucial that you test all cards to see that they go into low current sleep modes properly because the SD card uses 2-4x as much power as all the other components in your logger.
1 × Wires, resistors, leds, holders, connectors, etc ( $3-10 ) I splash out now on soft silicone wire, and pricey Deans connectors, but early builds used the scraps I had lying around and they worked fine.