xstatbar is a minimal system stats viewer for X11. It shows stats for CPU usage (with support for multi-core processors), memory usage, the number of processes, battery usage, volume, and time. It displays as a small, horizontal bar that integrates nicely with tiling window managers.
Python System Monitor (Psymon) is a cross-platform task and performance monitor. It features global process monitoring, system load history (CPU, memory, network, and disks), disk information, network connections, detailed information and CPU, and memory percentage history per process.