Monitoring and limiting internet usage I am currently having to access the internet through a wireless dongle which is giving me a 2gb per month download allowance. I need to be really cautious about my internet usage.

Just this afternoon I ran out due to some process downloading 600mb of something. I have no idea what it was (although I suspect iTunes) or why it would need to download so much.

Is there anyway I can monitor which processes are using the network, or failing that set up a firewall that only allows the apps I choose to have access?
If you install Little Snitch you can monitor which application using internet. Every time any application wants to use internet you must accept or deny it, but it doesn't have an ability to measure bandwidth usage of each application.

You can see its demo here.