Can I view system stats in the Terminal? The widget iStat Pro allows to monitor different system stats like


CPU usage
network bandwidth in/out 
memory usage
...


How can I view such system stats in the Terminal?
Depending on what you want displayed there are several options available using Terminal:

top: real-time sorted display of running processes such as memory and CPU usage
iostat: I/O summary statistics for terminal, device and CPU operations
vm_stat: Mach virtual memory statistics
diskutil list: disk/volume capacity
df: used/free space on mounted partitions
fs_usage: real-time file activity for both disk and network
nettop: updated information about the network (a bit like top for net I/O)
w: who is logged in and what they are doing; plus a brief system load summary
ifconfig and ipconfig: network interface and IP protocol details

Most of these commands have a huge list of options.  It's probably best to consult the man pages for details.
I usually rely on:

top -u -s 10 to identify CPU-hogging processes;
fs_usage -f filesys or fs_usage -f network to identify processes generating a lot of disk/network load.