Here is an under construction web site for Posting and searching for open source data logger projects. I'll get the data structures security set up over the weekend and will keep posting progress to this thread:
http://www.opensourceloggers.com/
It'd be very helpful to hear about what sort of catagories data loggers should fall under - such as Arduino, SQL, JDO etc - maybe the language, licences type etc. Posting a description, repository location...anything else?