Is there a network usage utility that shows data usage by network connection
If you are looking for simple real time statistics, checkout iStat Pro. 
It is a widget that provides a lot more than just network information.

http://www.apple.com/downloads/dashboard/status/istatpro.html

Surplus Meter or Net Monitor look like they would do exactly what you want:

http://mac.appstorm.net/roundups/internet-roundup/4-ways-to-monitor-bandwidth-usage-on-your-mac/