Citizen science participants in NestWatch used them for several years to gather temperature data from bird nests and nest boxes. Participants would program loggers, deploy them, and then download the data and email data files to researchers.
The Thermochron iButton measures only temperature and there are many styles http://bit.ly/SPIISn. The least expensive is only $3.80/logger, but it won't keep real time records; the cheapest that keeps time-temperature records is $14, if you buy bulk of 1000 loggers.
The Hydrochron iButton measures temperature and humidity at a bulk price of $55/logger if you order 1000 loggers. Here are the specs: http://bit.ly/UgYvrI
Step 1: Delivering the recorded data
The data loggers are re-usable. Depending on the sampling period needed at each site, it could be easy for participants to take turns using data loggers, sharing through shipping in the mail.