If you want to roll your own application/software,
www.microchip.com sells a USB evaluation board:
DM163025 - PICDEM FS USB
and the MMC/SD daughter board that can be used with the above board.
AC164122 - PICtail Daughter Board for SD & MMC Cards
I've had both for almost a year, but not enough time to test the data logging software out.
A pre-canned serial data logger is also available like the Acumen DataBridge SDR-OEM-SF. It records all serial port traffic onto SD but is pricey.
I'd like a totally passive data logger that I don't have to send commands to in order to get it to log/save the data.
For debugging purposes I need not only all the raw sensor data and outputs, but intermediate steps of the code in processing the data. Its usually one of the intermediate steps that is messing up and causing problems.
Bud