1: top - Process Activity Command
2: vmstat - System Activity, Hardware and System Information
3: w - Find Out Who Is Logged on And What They Are Doing
4: uptime - Tell How Long The System Has Been Running
5: ps - Displays The Processes
6: free - Memory Usage
7: iostat - Average CPU Load, Disk Activity
8: sar - Collect and Report System Activity
9: mpstat - Multiprocessor Usage
10: pmap - Process Memory Usage
11 and 12: netstat and ss - Network Statistics
13: iptraf - Real-time Network Statistics
14: tcpdump - Detailed Network Traffic Analysis
15: strace - System Calls
16: /Proc file system - Various Kernel Statistics
17: Nagios - Server And Network Monitoring
18: Cacti - Web-based Monitoring Tool
19: KDE System Guard - Real-time Systems Reporting and Graphing
20: Gnome System Monitor - Real-time Systems Reporting and Graphing
from : http://www.cyberciti.biz/tips/top-linux-monitoring-tools.html