Determine What Service Is Creating Traffic

I'm using MRTG to display graphs of traffic to my server, and today noticed a period of several hours where there was a lot of *incoming* data over the network, which is unusual. There's a good chance it's some harmless script or service that I've forgotten about, but really I need more information on what services/ports are creating what traffic, which brings me to my question.

I'm wondering whether there's an easy way to check what services (or ports) are creating traffic, and what volumes of traffic. It would be nice if it was possible to graph or breakdown the traffic by service somehow, and determine which are the biggest users of bandwidth and when. This is a dedicated CentOS server that I can install basically anything on, but that doesn't use a control panel, so I'd like to see what great ideas you folks can come up with, or whether there's any nice software you can suggest. I'm only using the server for hobbyist stuff at the moment, so free software, or ideas using built in commands are preferable.