Following in the spirit of Hidden Features of PowerShell and various others on Stack Overflow, what Linux commands or combinations of commands do you find essential in your work?
Also See:
Hidden Features of MySQL
Hidden Features of PowerShell
Hidden features of Oracle Database
Hidden Features of Windows 2008
Hidden Features of Solaris/OpenSolaris
Hidden Features of SQL Server
Hidden Features of IIS (6.0 / 7.0)