Where can I find statistics on download amount on OSX? I'd like to buy a mobile broadband service, but there's a limit in gigabyte per month I can download. 

Before i buy, it would be great to know what i currently consume, bandwidth wise. Is this statisti available anywhere on a vanilla OS X system? Should I get an additional application?
Activity Monitor will show your network usage for the current session, but this gets reset when you reboot. However, if you're just looking for a rough estimate of how much data you use before you get your mobile service, this may suffice. After you get your service, you will probably find that your carrier has a way of checking via a website.

Regarding bmike's answer, I believe he's talking about iOS.