NTM (Network Traffic Monitor) is a great application for those who have a limited internet plan. It can monitor the inbound, outbound and total traffic, the time spent online and various other very useful features for limited / mobile internet connections. Futher more, it can automatically disconnect your computer when the limit is reached (for cases when exceeding the limit is very expensive).
NTM also allows you to set the period you want to monitor and get a daily traffic report as well as daily averages.
More info and download (includes Ubuntu .deb)
Thanks to Vlad for the tip!