I've been told by my ISP I'm using too much bandwidth so after using Activity Monitor to see that I'm using ~2GB a day I've used the nettop command line utility to work out where. Turns out it's iTunes.
I don't use it for downloading/streaming music (other than podcasts of which I definitely don't have 2GB of new ones a day) or movies. What is iTunes doing?
This is on a late 2009 MacBook running Lion 10.7.5 with iTunes version 10.7
UPDATE: Genius (and store and home sharing) all disabled (today to try and track this down) and data still being gobbled - these things are pushing data out anyway right, I'm not using match, files are all local, not buying anything. Seems to be an intermittent thing, data being static while I'm looking but will have jumped by xMB next time I check.