Display load average, top CPU processes (use SHIFT M to sort by memory use).
top
Display a tree of all processes
pstree
Display all processes
ps -aux
Customise ps output to your requirements
ps -eo pid tid class rtprio ni pri stat comm
Find zombied processes
ps -eo pid stat | grep Z
Find processes waiting for disk
ps -eo pid stat | grep D
Trace execution of process
strace
Display free memory
free
Display virtual memory status
vmstat
Display inter-process communication status
ipcs -l or -a
Display names of slab memory in size order
slabtop