lsof

The author of the article below says he uses lsof most for getting network connection related information from a system, but netstat is just as useful and made specifically for this particular purpose.
lsof will do more than network connections, though. It will also tell you what files the program has open in addition to unix sockets and libraries. Of course you could also get all this from /proc but lsof makes it nice and easy. 'of' is actually for 'open files'.

http://dmiessler.com/study/nix/commands/lsof/
or
http://img231.imageshack.us/img231/5250/lsofhh8.png

You can use lsof to find out what is using your sound card (if it's FireFox, it's probably some YouTube video or some such multimedia piece of embedded Flash):

Other great commands you'll love: 'watch', 'strace', 'ltrace', 'iostat', 'vmstat', 'tcpdump', 'truss', 'netcat' (nc), 'wget'.
http://en.wikipedia.org/wiki/Watch_(Unix)
http://en.wikipedia.org/wiki/Strace
http://en.wikipedia.org/wiki/Iostat
http://en.wikipedia.org/wiki/Netstat

Just as useful or better network monitoring tools / options:
iptraf:
http://www.linuxhaxor.net/2007/08/27/iptra...ed-lan-monitor/
itrace:
http://www.linuxhaxor.net/2007/07/17/backt...ing-all-itrace/
intrace:
http://www.linuxhaxor.net/2007/07/16/backt...ng-all-intrace/
tcptrack:
http://www.linuxhaxor.net/2007/09/04/sniff...-with-tcptrack/
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This post has been edited by dserban: Jun 3 2008, 07:25 PM