There’s a great thread on ask ubuntu on seeing which applications are using the Internet. I’m going to add my own spin on the answers for future reference.

lsof -i | awk '{print $1}' | uniq | tail -n +2

Breaking it down…

• lsof -i shows us a lot of information pertaining to processes accessing the Internet.
• awk '{print $1}' filters the last output to only shown us the COMMAND column.
• uniq filters out multiple occurrences of a single application.
• tail -n +2 removes the first COMMAND line.