Usefull Unix commands, tips and tricks...

Have you seen this question on Hacker News? It's great and actually there are really some useful commands and tips for use of the command line more efficiently. Here I'll list some of them in case you want a quick glance, but be aware that something mentioned there might already be known to me and/or I decided that it's not so important and excluded it.

So, without further ado, here's my list of favorites: