Linux – In linux, how can you tell which processes use the most network bandwidth

You could use http://nethogs.sourceforge.net/

You might also want to look through the answers given at https://serverfault.com/questions/44492/ip-process-based-bandwidth-usage-stats-tool-for-linux

You can use

iptraf

or

iftop -P

to identify connections that consume much of your bandwidth. A connection is identified by a pair of (localip:port, remoteip:port). You can then use

netstat -tunp

to look up what process uses that connection. Mostly however, you will be able to tell what service is responsible only by looking at the port, e.g. if the connection uses localip:80 on the local side its probably your webserver ;-)