In the same vein as the "Useful Command-line" questions (for Windows, Linux, and Mac) , I think it would be handy to have "useful ways to use utility x" questions. Man pages tell you what parameters do, but not necessarily why you would use them, what the result means, what useful things the command does that you'd never know without extensive experimentation, or how to get the answer you really want.
I'd like to know about netstat. It would appear that I should be able to figure which processes are using bandwidth, and, indeed, how fast the system is using bandwidth. It also looks useful for detecting unwanted connections (likely virii), and it gives all sorts of routing information (that I only had to play with when trying to make a Sharp Zaurus PDA use TCP/IP over USB.) In other words, it sounds like a gold mine, and I was hoping some of you would share nuggets of information you've found.
Please include the version of netstat and your OS in your reply. It would be nice to see some sample output and know what it means. I've marked this question as community wiki, and I hope you'll do the same in your answers, so that other people, knowing a different OS, can put down a near equivalent command if they know, in the same answer, and then we can vote on which answers are the most useful.