There are a number of PC apps that do the trick, I've read trying to find one for mac. But none seem to do it for mac (Mountain Lion).
I have Little Snitch already and tried putting
lsof -P -i -n | cut -f 1 -d " " | uniq into the Terminal, but both of these methods only shows what is using internet right at this moment. It doesn't, so to say, sum up all the data usage for the apps.