I'm not sure where to get it, but at school we used to use scientific data loggers that you could let run for days, then plug into a computer and trasfer all the data. From memory they were worth about $1000 each, but that was 5 years ago, they must've come down by now. It was also a pretty fancy unit, had all sorts of capabilites, could plug light gates and photosensors and whatnot into it.
I recall someone on here having an electronic chart of their fermenting temps...
Does anyone know how to set something like this up to track boil/mash temps as well?
There was another thread on here recently about the DS1820 sensors. the rrdtool software is free and works on either Linux or Windows. The pic shows the temperature of our chilled water tank, but I have others sensing the air temperature, HLT, fermenters and coolroom.
They cost $50 come with a temp probe, and will plug into your computer.
Another alternative is a RS232 based 10-bit data logger for $25. DI-194 But you would need to find a wire up some thermo couples.
For me, the DS1820s really are great. You can see a link to a ~$40 Ozitronics kit from Groggy's site if you are not handy with a soldering iron. Otherwise you can build up a DB9 plug and order some free samples from Dallas Semi and away you go.