Processes using the internet

Sometimes I notice that some mysterious process is downloading data from the internet. (gkrellm shows me that data is being downloaded, even though I'm not doing anything). This is probably just "apt-get update" running in the background. But is there any way to tell for sure which process are sending and receiving data from the internet?

I asked this question on Ubuntuforums, and I got a very nice reply.

There are two ways of doing this:

1. Simply use the command: sudo lsof -Pni
   
2. You can use a tool called trojanscan (homepage here)
   

Personally, I prefer the first method. But system administrators may like TrojanScan better, has it has more features, and you can choose to ignore the usual processes.

Have fun

-Aku