I feel like I'm repeating myself, but the data logger I use is this one:
It is open source, it uses 1-wire sensors (Travis tested it with up to 70 sensors, could have gone higher), it is very reliable, it logs to an SD card (I tried a 16GB card, and it works fine), it is cheap $45 complete and ready to go.
I got mine
HERE.
Since it is an Arduino, the code can be modified but you need a cheap FTDI Basic Breakout board to talk to it.
I've had mine for some years now, with no problems at all. It does everything I want it to do.
I'm currently monitoring my whole house with one.
-AC