As a sysadmin. I don't love any other tool nearly as much as I love atop. It comes close to making all other interactive general-purpose performance monitoring tools redundant. The logging, too, is fantastic. There's nothing like looking through the graphs of my metrics gathered by collectd, wondering why a certain spike or pattern occurred, and being able to step through previous days per-process resource usage via atop and figure it out. There's a great introduction to it at https://lwn.net/Articles/387202/