How do I find out which process is eating up my bandwidth?

I've had a lot of success with nethogs. It has to run as root but there are different ways you can sort the statistics (like KB/s or total bandwidth monitored since nethogs started).

Also, if you use wireless you need to pass the device to it.

Install it with command: sudo apt-get install nethogs

Example: sudo nethogs wlan0

Use iftop to locate the TCP port on your machine that is receiving the most traffic. Then use sudo netstat -tup to locate the process "owning" that port.

That's the process you're looking for.

PS: Should work for UDP too.

You might want to look into ntop - which should monitor network activity on a process level. You can find ntop in the Software Center or with sudo apt-get install ntop

For installation instruction, follow their page http://packages.ntop.org/