The Logomatic is a great data logger that appears as a flash drive on any operating system. Logs are created in FAT32 format on the microSD media and can be downloaded quickly over a USB connection by dragging and dropping the text files from the device. The board has a built-in RTC and 10 available GPIO pins.
The hardware is released under Creative Commons ShareAlike 4.0 International. The code is beerware; if you see me (or any other SparkFun employee) at the local, and you've found our code helpful, please buy us a round!
Distributed as-is; no warranty is given.