Network interface monitoring. Linux command line.
Finally, the first tool which runs on Linux and not on Windows 🙂 Iftop is a bit like top, except that instead of monitoring CPU usage by process, it monitors network usage by connection (from IP => to IP). It runs in the console, so it’s great when you need a quick view of what network activity is going on on your server, as long as you don’t have too many connections (then it becomes a bit tedious to browse).
There isn’t much more to say, actually, apart from the quite surprising fact that it’s not bundled in a default Linux installation (at least not on Debian or OpenSUSE), so you have to install it manually:
zypper install iftop or
apt-get install iftop. Also, it seems it can only be run as root.