Nigel's monitor(link), dubbed "Nmon", is a fantastic tool for monitoring, recording and analyzing a Linux/*nix system's performance over time. Nmon was originally developed by IBM and Open Sourced in the summer of 2009. By now Nmon is available on just about every linux platfrom and architechture. It provides a great real-time command line visualization of current system statistics, such as CPU, RAM, Network and Disk I/O. However, Nmon's greatest feature is the capability to record system performance snapshots over time. For example: "nmon -f -s 1".