I've been looking to compile a list of most useful one-liner commands recently. Personally, I think it would be helpful for a lot of people if they could simply browse through the answers and discover other clever one-liners from other users.
Answers would have to be strictly one line commands (piping is of course permitted) and nothing too lengthy (simply putting a small script into one line would not be an answer). Some examples:
Finding information about a running process:
ps -ef | grep <process-name>
List mounted filesystems in a nice way:
mount | column -t
Of course, there are more but I'll save the good ones for the question if it isn't off topic :)