Top

top will continuously show your machine's load and which programs are taking what percentage of resources, in real time.

Main commands in top:

s will change the delay (in seconds) between two refreshes of top. Split-seconds aren't 
  allowed but for quick refresh you can use zero.
o followed by one of the following changes the sorting of the display by time of: TIME, 
  RES, SIZE, CPU, PRI
k will send signals such as kill to specified tasks.
m toggles the display between CPU and I/O mode.
u show only the processes of a certain user
r renice a process for altering its speed. Some renice acts are restricted to root 
  or su mode.
S toggle the display of system processes
t toggle the display of top itself
d change the number of time to refresh top before ending it.
q (at the command line only as "top -q", restricted to root only) to renice top to -20 
  for extra fast refresh

Most of the commands work the same way in the command line.