Linux Server Performance Monitoring

I'm looking to monitor performance on my Linux servers (which happen to be Centos). What are the best tools for monitoring things in realtime such as:

• Disk Performance I/O, swapping etc..
• CPU Performance

Looking for low level tools, rather than web based tools such as Nagios, Ganglia etc...

n.b. I'd like to know exactly what each tool does rather than just having a list of random toolnames if possible please. Why the tool is a better option than others would be good also.

collectl is a great performance monitor with some unique and advanced features like InfiniBand and Lustre support or environmental and NFS monitoring. Of course it does basic stuff like disk and cpu monitoring, too. Highly recommended!

iostat, sar, top, munin