Comment by profquail

These tools aren't limited to Linux; if you're running FreeBSD:

* http://www.freshports.org/sysutils/htop/

* http://www.freshports.org/sysutils/py-glances/

* http://www.freshports.org/sysutils/apachetop/

Some others mentioned in the comments:

* http://www.freshports.org/net-mgmt/iftop/

* http://www.freshports.org/net-mgmt/bwm-ng/

* http://www.freshports.org/sysutils/xsysstats/

* http://www.freshports.org/sysutils/atop/

If you're running OS X / FreeBSD / Solaris, there are many useful DTrace scripts for system monitoring and profiling:

http://www.brendangregg.com/dtrace.html