cpumon - A trivial CPU performance monitor written in Tcl/Tk.

lsof - Lsof lists files open to Unix processes and runs on a wide range of Unix dialects.

Mfsm - A X-Windows Motif utility that monitors free space and user quotas of filesystems. The program displays a drive type icon, a usage bar chart, and an alert bell toggle.

Proctool - GUI, enhanced version of "top". Shows most active processes in real time.

SymbEL (SE) - An interpreted language that provides an extensive toolkit for building performance tools and utilities. If you are fed up with the limitations of vmstat, iostat and sar, then this is the tool for you. (Info page, what it's about.)

top - Process monitor program.

top - Process monitor program -Alternate site. (Site #2/UK)

xcpustate - A snapshot performance monitor.